Liquid-jet device, image forming apparatus, and method for adjusting landing positions of liquid droplets

BACKGROUND

1. Technical Field

This disclosure generally relates to a liquid jet device, an image forming apparatus, and a method for adjusting landing positions of liquid droplets.

2. Description of the Related Art

There are image forming apparatuses (e.g., a printer, a fax machine, a copier, and a multifunction copier having functions of a printer, a fax machine, and a copier) that use a liquid-jet device including a recording head implemented by a liquid-jet head to form (record or print) an image on paper (not limited to a sheet of paper but also refers to any medium on which an image can be formed, and may also be called a recording medium, recording paper, recording sheet, recording material, etc.). A liquid-jet device jets droplets of a recording liquid (or ink) from a liquid-jet head onto paper being carried in an image forming apparatus and thereby forms an image on the paper.

In the present application, an image forming apparatus refers to an apparatus that forms an image by jetting a liquid onto a recording medium made of paper, thread, fabric, silk, leather, metal, plastic, glass, wood, ceramic, etc. Also, “image forming” indicates not only a process of forming an image such as a character or a figure having a meaning on a recording medium, but also a process of forming a meaningless image such as a pattern on a recording medium. In other words, an image forming apparatus may even refer to a textile printer or an apparatus for forming a metal wiring pattern. Liquids used in an image forming apparatus are not limited to a recording liquid and ink. Further, a liquid-jet device refers to any device that jets a liquid from its liquid-jet head. The use of a liquid-jet device is not limited to image forming.

In a liquid-jet device or an image forming apparatus, a carriage having a recording head is moved forward (forward scan) and backward (backward scan) and recording (or printing) is performed in both the forward and backward directions (bidirectional printing). When printing lines with such a liquid-jet device or an image forming apparatus, misalignment tends to occur between lines printed by the forward and backward scans.

To solve this problem, some inkjet recording apparatuses have a line-adjustment function for adjusting the positions of lines. With a line-adjustment function, for example, the user prints a test chart and enters an adjustment value based on the results on the printed test chart to adjust the timing of jetting ink. However, selection of the adjustment value varies between users and depends on the ability of the user. If an incorrect adjustment value is entered, it may worsen the problem.

Patent document 1 discloses a liquid-jet image forming apparatus having a function to correct image density irregularity. In the disclosed image forming apparatus, a test pattern is printed on a recording medium or a conveyor belt, color data of the test pattern are obtained by scanning, and drive conditions for a recording head are adjusted based on the obtained color data to correct image density irregularity. <ul><li id="ul0001-0001" num="0009">[Patent document 1] Japanese Patent Application Publication No. 4-39041</li></ul>

Patent document 2 discloses an inkjet recording apparatus capable of detecting a defective nozzle of a liquid-jet head. In the disclosed inkjet recording apparatus, a test pattern composed of dots of different colors is formed using a cyan ink, a magenta ink, and a yellow ink in an area on a recording medium conveying part, the test pattern is scanned by an RGB sensor, and a defective nozzle is determined based on the scanned test pattern. <ul><li id="ul0002-0001" num="0011">[Patent document 2] Japanese Patent No. 3838251</li></ul>

Patent document 3 discloses an inkjet recording apparatus having a calibration function. In the disclosed inkjet recording apparatus, a test pattern composed of one or more of a nozzle clogging detection pattern for detecting nozzle clogging, a color shift detection pattern for detecting a color shift, and a head position adjustment pattern for adjusting the position of a recording head is formed on a part of a conveyor belt, the formed test pattern is scanned using an imaging device such as a charge-coupled device (CCD), and calibration is performed based on the scanned test pattern. <ul><li id="ul0003-0001" num="0013">[Patent document 3] Japanese Patent Application Publication No. 2005-342899</li></ul>

Patent document 4 discloses an electrophotographic image forming apparatus capable of detecting the density of toner images formed on a photosensitive drum using a sensor. The sensor includes a light-emitting element for illuminating the toner images, a light-receiving element for receiving specularly reflected light from the toner images, and a light-receiving element for receiving diffusely reflected light from the toner images. With the sensor, the image forming apparatus can detect the density of toner images having different characteristics. <ul><li id="ul0004-0001" num="0015">[Patent document 4] Japanese Patent Application Publication No. 5-249787</li></ul>

Patent document 5 discloses a method of determining the amount of adhering toner based on detection results from a sensor capable of detecting both specularly reflected light and diffusely reflected light from a toner image. <ul><li id="ul0005-0001" num="0017">[Patent document 5] Japanese Patent Application Publication No. 2006-178396</li></ul>

According to technology disclosed in patent documents 1 through 3 described above, a test pattern is formed on a conveyor belt and the formed test pattern is scanned to obtain its color data based on which various adjustments are made. One problem with the disclosed technology is that if the color of an ink is similar to that of the conveyor belt, it becomes difficult to obtain accurate color data of a test pattern. One way to obviate this problem is to use light sources with different wavelengths corresponding to respective colors. However, this method increases the cost of a detecting unit or an imaging unit for obtaining color data of a test pattern. For example, there is a conveyor belt implemented by an electrostatic belt comprising an insulating layer on the upper side and a medium-resistance layer containing carbon for adjusting electrical conductivity on the back side. Since such a conveyor belt has a black color similar to that of a black ink, it is difficult to correctly detect a black part of a test pattern based solely on reflected light from the test pattern or by scanning the test pattern with an imaging unit.

More specifically, with the image forming apparatus disclosed in patent document 1, since a test pattern formed on a recording medium conveying part is scanned by a sensor, it is difficult to obtain accurate color data of the test pattern if the color of an ink used to form the test pattern is similar to that of the conveying part. Thus, the disclosed configuration makes it necessary to provide a filter for each color and therefore increases the production cost. In the inkjet recording apparatus disclosed in patent document 2, an RGB sensor is used to scan a test pattern formed on a recording medium conveying part. Also with this configuration, it is difficult to obtain accurate color data of the test pattern if the color of an ink used to form the test pattern is similar to that of the conveying part. Therefore, to improve the accuracy of the color data, it is necessary to limit the colors of inks used with the recording medium conveying part. Also, since a laser beam used by the RGB sensor scans extremely small spots one by one, the result of scanning tends to be affected by a tiny foreign object or a flaw on the conveying part. Further, an RGB sensor requires a light-receiving element for each color and is therefore expensive. In the inkjet recording apparatus disclosed in patent document 3, an imaging device is used to scan a test pattern formed on a recording medium conveying part. With this configuration, it is difficult to obtain accurate color data of the test pattern if the color of an ink used to form the test pattern is similar to that of the conveying part. Also, since the imaging device recognizes the test pattern as a two-dimensional image, a processing system with higher performance than that for processing a one-dimensional image is necessary. This in turn increases the cost of the inkjet recording apparatus.

To obviate the above problems, research is being conducted to apply the method disclosed in patent documents 4 and 5 for detecting the density of toner images or the amount of adhering toner to a liquid-jet image forming apparatus. Since the shape of toner particles does not change even when they are brought into contact with each other, it is possible to form a test pattern by heaping up toner in the form of a line and to accurately scan the test pattern. However, if this method is applied to a liquid-jet image forming apparatus without change, it is not possible to accurately scan a test pattern since liquid droplets clump together.

Meanwhile, a method where a test pattern is formed by jetting ink droplets onto plain paper and is scanned by an optical sensor also has a problem. With this method, bleeding caused by penetration of ink into the plain paper results in a blurred test pattern and makes it difficult to accurately detect the landing positions of ink droplets (positions of jetted ink droplets on a target surface).

BRIEF SUMMARY

In an aspect of this disclosure, there is provided a liquid-jet device that includes a liquid-jet head configured to jet liquid droplets; a pattern formation control unit configured to control the liquid-jet head and thereby to form a test pattern composed of separate liquid droplets on a water-repellent part; a detecting unit including a light-emitting element configured to illuminate the test pattern on the water-repellent part and a light-receiving element configured to receive specularly reflected light from the illuminated test pattern and to output a detection signal proportional to the received specularly reflected light; and a landing position adjusting unit configured to adjust landing positions of the liquid droplets based on the detection signal from the light-receiving element.

In another aspect, there is provided an image forming apparatus for forming an image on a recording medium. The image forming apparatus includes a water-repellent conveyor belt configured to convey the recording medium, and a liquid-jet device. The liquid-jet device includes a liquid-jet head configured to jet liquid droplets; a pattern formation control unit configured to control the liquid-jet head and thereby to form a test pattern composed of separate liquid droplets on the conveyor belt; a detecting unit including a light-emitting element configured to illuminate the test pattern on the conveyor belt and a light-receiving element configured to receive specularly reflected light from the illuminated test pattern and to output a detection signal proportional to the received specularly reflected light; and a landing position adjusting unit configured to adjust landing positions of the liquid droplets based on the detection signal from the light-receiving element.

BRIEF DESCRIPTION OF THE DRAWINGS

<figref idrefs="DRAWINGS">FIG. 1</figref> is a schematic diagram illustrating a configuration of an image forming apparatus <b>200</b> according to an embodiment of the present invention;

<figref idrefs="DRAWINGS">FIG. 2</figref> is a plan view of an image forming unit and a paper conveying unit of the image forming apparatus <b>200</b>;

<figref idrefs="DRAWINGS">FIG. 3</figref> is an elevational view of the image forming unit and the paper conveying unit shown in <figref idrefs="DRAWINGS">FIG. 2</figref>;

<figref idrefs="DRAWINGS">FIG. 4</figref> is a cut-away side view of a conveyor belt;

<figref idrefs="DRAWINGS">FIG. 5</figref> is a block diagram illustrating a configuration of a control unit of the image forming apparatus <b>200</b>;

<figref idrefs="DRAWINGS">FIG. 6</figref> is a block diagram illustrating an exemplary mechanism for detecting and adjusting landing positions of liquid droplets according to a first embodiment of the present invention;

<figref idrefs="DRAWINGS">FIG. 7</figref> is a drawing illustrating the exemplary mechanism for detecting and adjusting landing positions of liquid droplets in more detail;

<figref idrefs="DRAWINGS">FIG. 8</figref> is a drawing illustrating an exemplary test pattern formed on a conveyor belt;

<figref idrefs="DRAWINGS">FIG. 9</figref> is a drawing illustrating an image sensor;

<figref idrefs="DRAWINGS">FIG. 10</figref> is a drawing illustrating diffusely reflected light from a liquid droplet;

<figref idrefs="DRAWINGS">FIG. 11</figref> is a drawing illustrating diffusely reflected light from a flattened liquid droplet;

<figref idrefs="DRAWINGS">FIG. 12</figref> is a graph showing the relationship between the time elapsed after a liquid droplet is placed on a target surface and a sensor output voltage;

<figref idrefs="DRAWINGS">FIG. 13</figref> is a drawing illustrating a test pattern according to an embodiment of the present invention;

<figref idrefs="DRAWINGS">FIG. 14</figref> is a drawing illustrating a test pattern of a comparative example;

<figref idrefs="DRAWINGS">FIG. 15</figref> is a drawing illustrating a test pattern formed with toner;

<figref idrefs="DRAWINGS">FIGS. 16A and 16B</figref> are drawings used to describe a first exemplary position detecting process;

<figref idrefs="DRAWINGS">FIGS. 17A and 17B</figref> are graphs used to describe a second exemplary position detecting process;

<figref idrefs="DRAWINGS">FIG. 18</figref> is a drawing used to describe a third exemplary position detecting process;

<figref idrefs="DRAWINGS">FIG. 19</figref> is a drawing illustrating a first exemplary arrangement of liquid droplets forming a test pattern;

<figref idrefs="DRAWINGS">FIGS. 20A and 20B</figref> are drawings illustrating second exemplary arrangements of liquid droplets forming a test pattern;

<figref idrefs="DRAWINGS">FIGS. 21A and 21B</figref> are drawings illustrating third exemplary arrangements of liquid droplets forming a test pattern;

<figref idrefs="DRAWINGS">FIGS. 22A through 22C</figref> are drawings illustrating other exemplary arrangements of liquid droplets forming a test pattern;

<figref idrefs="DRAWINGS">FIG. 23</figref> is a drawing used to describe a contact area of liquid droplets in a detection range;

<figref idrefs="DRAWINGS">FIG. 24</figref> is a graph showing the relationship obtained by an experiment between the proportion of diffuse reflection area of liquid droplets and a detection result;

<figref idrefs="DRAWINGS">FIG. 25</figref> is a drawing illustrating a liquid droplet and used to describe a pattern diffuse reflection rate;

<figref idrefs="DRAWINGS">FIG. 26</figref> is a drawing illustrating a contact angle of a liquid droplet;

<figref idrefs="DRAWINGS">FIGS. 27A through 27D</figref> are drawings illustrating block patterns;

<figref idrefs="DRAWINGS">FIG. 28</figref> is a drawing illustrating a line misalignment test pattern;

<figref idrefs="DRAWINGS">FIGS. 29A and 29B</figref> are drawings illustrating color misalignment test patterns;

<figref idrefs="DRAWINGS">FIG. 30</figref> is a drawing illustrating an exemplary arrangement of test patterns on a conveyor belt;

<figref idrefs="DRAWINGS">FIG. 31</figref> is a flowchart showing an exemplary process of adjusting landing positions of liquid droplets;

<figref idrefs="DRAWINGS">FIG. 32</figref> is a plan view of an image forming unit and a paper conveying unit according to a second embodiment of the present invention;

<figref idrefs="DRAWINGS">FIG. 33</figref> is a drawing illustrating a third embodiment of the present invention;

<figref idrefs="DRAWINGS">FIG. 34</figref> is a drawing illustrating a fourth embodiment of the present invention;

<figref idrefs="DRAWINGS">FIG. 35</figref> is another drawing illustrating the fourth embodiment of the present invention;

<figref idrefs="DRAWINGS">FIGS. 36A and 36B</figref> are drawings illustrating a retracting mechanism according to the fourth embodiment;

<figref idrefs="DRAWINGS">FIG. 37</figref> is a flowchart showing an exemplary process according to the fourth embodiment;

<figref idrefs="DRAWINGS">FIG. 38</figref> is a drawing illustrating a fifth embodiment of the present invention;

<figref idrefs="DRAWINGS">FIG. 39</figref> is a drawing illustrating a sixth embodiment of the present invention;

<figref idrefs="DRAWINGS">FIG. 40</figref> is a drawing illustrating a seventh embodiment of the present invention;

<figref idrefs="DRAWINGS">FIG. 41</figref> is a drawing illustrating an eighth embodiment of the present invention;

<figref idrefs="DRAWINGS">FIG. 42</figref> is a flowchart showing an exemplary process according to a ninth embodiment;

<figref idrefs="DRAWINGS">FIG. 43</figref> is a drawing illustrating a tenth embodiment of the present invention;

<figref idrefs="DRAWINGS">FIG. 44</figref> is a flowchart showing an exemplary process according to the tenth embodiment;

<figref idrefs="DRAWINGS">FIG. 45</figref> is a perspective view of a cleaning roller according to an eleventh embodiment of the present invention;

<figref idrefs="DRAWINGS">FIG. 46</figref> is a flowchart showing an exemplary process according to a twelfth embodiment of the present invention;

<figref idrefs="DRAWINGS">FIG. 47</figref> is a flowchart showing an exemplary process according to a fifteenth embodiment of the present invention;

<figref idrefs="DRAWINGS">FIG. 48</figref> is a drawing illustrating a sixteenth embodiment of the present invention;

<figref idrefs="DRAWINGS">FIG. 49</figref> is a flowchart showing an exemplary process according to the sixteenth embodiment; and

<figref idrefs="DRAWINGS">FIG. 50</figref> is a flowchart showing an exemplary process according to a seventeenth embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention are described below with reference to the accompanying drawings. An image forming apparatus including a paper conveying unit (recording medium conveying unit) according to an embodiment of the present invention is described below with reference to <figref idrefs="DRAWINGS">FIGS. 1 through 5</figref>. <figref idrefs="DRAWINGS">FIG. 1</figref> is a schematic diagram illustrating a configuration of an image forming apparatus <b>200</b>. <figref idrefs="DRAWINGS">FIG. 2</figref> is a plan view of an image forming unit and a paper conveying unit of the image forming apparatus <b>200</b>. <figref idrefs="DRAWINGS">FIG. 3</figref> is an elevational view of the image forming unit and the paper conveying unit shown in <figref idrefs="DRAWINGS">FIG. 2</figref>.

The image forming apparatus <b>200</b> includes a body <b>1</b>, and includes an image forming unit <b>2</b> (may also be called a liquid-jet device) for forming an image, a paper conveying unit (recording medium conveying unit) <b>3</b>, a paper feeding unit <b>4</b>, and a paper ejecting unit <b>6</b> in the body <b>1</b>. In the image forming apparatus <b>200</b>, paper sheets <b>5</b> (may also be called recording media, and the material is not limited to paper) are fed one by one from the paper feeding unit <b>4</b> at the bottom of the case, the paper conveying unit <b>3</b> conveys the paper sheet <b>5</b> intermittently in a position facing the image forming unit <b>2</b>, the image forming unit <b>2</b> jets liquid droplets onto the paper sheet <b>5</b> being conveyed and thereby forms (records) an image, then the paper ejecting unit <b>6</b> ejects the paper sheet <b>5</b> onto a paper catch tray <b>7</b> on the upper side of the body <b>1</b>. The image forming unit <b>2</b> and the paper conveying unit <b>3</b> are integrated as an imaging engine unit <b>100</b> that is attachable to and detachable from the body <b>1</b>.

The image forming apparatus <b>200</b> also includes an image scanning unit <b>11</b> for scanning an image. The image scanning unit <b>11</b> is disposed above the paper catch tray <b>7</b> of the body <b>1</b> and is used to input image data (print data) to be formed by the image forming unit <b>2</b>. The image scanning unit <b>11</b> includes a scanning optical system <b>15</b> including a light source <b>13</b> and a mirror <b>14</b>; a scanning optical system <b>18</b> including mirrors <b>16</b> and <b>17</b>; a contact glass <b>12</b>; a lens <b>19</b>; and an imaging element <b>20</b> behind the lens <b>19</b>. The scanning optical system <b>15</b> and the scanning optical system <b>18</b> move and scan a document on the contact glass <b>12</b>, and the imaging element <b>20</b> converts the optical image of the scanned document into an image signal. The image signal is digitized and processed, and an image is printed according to the processed image signal. The image scanning unit <b>11</b> also includes a pressing plate <b>10</b> above the contact glass <b>12</b> to hold down a document.

As shown in <figref idrefs="DRAWINGS">FIG. 2</figref>, the image forming unit <b>2</b> includes a carriage guide rod <b>21</b> used as a primary guide part and disposed between a front board <b>101</b>F and a rear board <b>101</b>R, a guide stay <b>22</b> (see <figref idrefs="DRAWINGS">FIG. 3</figref>) used as a secondary guide part and disposed near a rear stay <b>101</b>B, a carriage <b>23</b> supported by the carriage guide rode <b>21</b> and the guide stay <b>22</b> so as to be movable in the main-scanning direction (carriage-scanning direction), and a main-scanning motor <b>27</b>. The main-scanning motor <b>27</b> moves the carriage <b>23</b> in the main-scanning direction via a timing belt <b>29</b> stretched between a drive pulley <b>28</b>A and a driven pulley <b>28</b>B.

The carriage <b>23</b> comprises recording heads <b>24</b><i>k</i><b>1</b> and <b>24</b><i>k</i><b>2</b> each implemented by a liquid-jet head for jetting a black (K) ink, and recording heads <b>24</b><i>c</i>, <b>24</b><i>m</i>, and <b>24</b><i>y </i>implemented, respectively, by liquid-jet heads for jetting cyan (C), magenta (M), and yellow (Y) inks (the recording heads may be collectively called recording head(s) <b>24</b> for brevity when color distinction is not important). The image forming unit <b>2</b> is a shuttle type where an image is formed by moving the carriage <b>23</b> in the main-scanning direction and jetting ink droplets from the recording heads (liquid-jetting units) <b>24</b> while the paper sheet <b>5</b> is carried in the sub-scanning direction by the paper conveying unit <b>3</b>.

The carriage <b>23</b> also includes sub-tanks <b>25</b> for supplying corresponding color inks to the recording heads <b>24</b>. Referring back to <figref idrefs="DRAWINGS">FIG. 1</figref>, the body <b>1</b> includes a cartridge holder <b>26</b>A for detachably holding ink cartridges (recording liquid cartridges) <b>26</b> containing, respectively, a black (K) ink, a cyan (C) ink, a magenta (M) ink, and a yellow (Y) ink. The inks (recording liquids) are supplied from the ink cartridges <b>26</b> to the corresponding sub-tanks <b>25</b> via tubes (not shown). The ink cartridges <b>26</b> can be inserted into the cartridge holder <b>26</b>A from the front side of the body <b>1</b>. The black ink is supplied from one of the ink cartridges <b>26</b> to two sub-tanks <b>25</b> corresponding to the recording heads <b>24</b><i>k</i><b>1</b> and <b>24</b><i>k</i><b>2</b>.

As the recording head <b>24</b>, one of the following three types, which employ different types of pressure-generating units (actuator units) for pressurizing ink in an ink channel (pressure generating chamber), may be used: a piezoelectric type employing a piezoelectric element that causes ink droplets to be discharged by deforming a vibrating plate forming a wall of the ink channel and thereby changing the volume of the ink channel; a thermal type employing a heat element that heats ink in the ink channel to generate air bubbles and causes ink droplets to be discharged by the pressure of the air bubbles; and an electrostatic type that includes an electrode facing a vibrating plate forming a wall of the ink channel and causes ink droplets to be discharged by deforming the vibrating plate with an electrostatic force generated between the vibrating plate and the electrode and thereby changing the volume of the ink channel.

Referring to <figref idrefs="DRAWINGS">FIGS. 2 and 3</figref>, a linear scale <b>128</b> having slits is provided along the main-scanning direction between the front board <b>101</b>F and the rear board <b>101</b>R. An encoder sensor <b>129</b> implemented by a transmissive photosensor for detecting slits of the linear scale <b>128</b> is attached to the carriage <b>23</b>. The linear scale <b>128</b> and the encoder sensor <b>129</b> constitute a linear encoder for detecting the position of the carriage <b>23</b>.

Also, as shown in <figref idrefs="DRAWINGS">FIG. 2</figref>, an image sensor <b>401</b> (detecting unit) for detecting positional deviation (deviation from correct landing positions) of ink (or liquid) droplets is attached to a side of the carriage <b>23</b>. The image sensor <b>401</b> is implemented by a reflective photosensor including a light-emitting element and a light-receiving element, and scans a test pattern formed on a water-repellent conveyor belt <b>31</b> (water-repellent part) and used to detect positional deviation of ink droplets.

Further, as shown in <figref idrefs="DRAWINGS">FIG. 2</figref>, a maintenance/cleaning mechanism <b>121</b> is provided in a non-image-forming area on one side of the carriage <b>33</b> with respect to the main-scanning direction. The maintenance/cleaning mechanism <b>121</b> maintains and cleans the nozzles of the recording heads <b>24</b>. The maintenance/cleaning mechanism <b>121</b> includes caps for covering nozzle surfaces <b>24</b><i>a </i>of the recording heads <b>24</b>. The maintenance/cleaning mechanism <b>121</b> includes a moisture-retention/suction cap <b>122</b><i>a</i>, four moisture retention caps <b>122</b><i>b </i>through <b>122</b><i>e</i>, a wiper blade <b>124</b> for wiping the nozzle surfaces <b>24</b><i>a</i>, and a waste-ink receiver <b>125</b> for receiving ink used to purge dried ink from nozzles of the recording heads <b>24</b>. In a non-image-forming area on the other side of the carriage <b>33</b> with respect to the main-scanning direction, a waste-ink receiver <b>126</b> is provided. The waste-ink receiver <b>126</b> is used to receive ink used to purge dried ink from the nozzles of the recording heads <b>24</b>. The waste-ink receiver <b>126</b> has openings <b>127</b><i>a </i>through <b>127</b><i>e. </i>

The paper conveying unit <b>3</b> includes a conveying roller <b>32</b> used as a drive roller; a driven roller <b>33</b> used as a tension roller; and an endless conveyor belt <b>31</b> stretched between the conveying roller <b>32</b> and the driven roller <b>33</b>. The conveyor belt <b>31</b> changes the direction of the paper sheet <b>5</b> fed from the paper feeding unit <b>4</b> approximately 90 degrees and then conveys the paper sheet <b>5</b> in a position facing the image forming unit <b>2</b>. The paper conveying unit <b>3</b> also includes a charging roller <b>34</b> to which an AC bias voltage for charging the surface of the conveyor belt <b>31</b> is applied; a platen guide <b>35</b> for guiding the conveyor belt <b>31</b> in an area facing the image forming unit <b>2</b>; a first pressing roller (entrance pressing roller) <b>36</b> for pressing the paper sheet <b>5</b> against the conveyor belt <b>31</b> in a position facing the conveying roller <b>32</b>; a second pressing roller (edge pressing roller) <b>37</b> disposed between the conveying roller <b>32</b> and the recording heads <b>24</b> and used to press the paper sheet <b>5</b> against the conveyor belt <b>31</b> in a position facing the platen guide <b>35</b>; a holding part <b>136</b> for holding the first pressing roller <b>36</b> and the second pressing roller <b>37</b>; and separating claws <b>39</b> for separating the paper sheet <b>5</b>, on which an image has been formed by the image forming unit <b>2</b>, from the conveyor belt <b>31</b>.

The conveyor belt <b>31</b> is turned in the paper conveying direction (sub-scanning direction) shown in <figref idrefs="DRAWINGS">FIG. 2</figref> by the conveying roller <b>32</b> that is rotated by a sub-scanning motor <b>131</b>, implemented by a DC brushless motor, via a timing belt <b>132</b> and a timing roller <b>133</b>. In this embodiment, as shown in <figref idrefs="DRAWINGS">FIG. 4</figref>, the conveyor belt <b>31</b> comprises an outside layer <b>31</b>A that attracts the paper sheet <b>5</b> and an inside layer (medium-resistance layer or earth layer) <b>31</b>B. The outside layer <b>31</b>A is made of a pure resin material, such as an ethylene-tetrafluoroethylene (ETFE) pure material, that is not resistance-adjusted. The inside layer <b>31</b>B is made of a material prepared by adjusting the resistance of the material of the outside layer <b>31</b>A with carbon. Alternatively, the conveyor belt <b>31</b> may be composed of one layer, or three or more layers.

A paper dust removing part <b>191</b> made of a polyethylene terephthalate (PET) film or Mylar (DuPont) is provided between the driven roller <b>33</b> and the charging roller <b>34</b>. The paper dust removing part <b>191</b> is in contact with the surface of the conveyor belt <b>31</b> and removes paper dust being carried on the conveyor belt <b>31</b> from the upstream. Also, a cleaning brush <b>192</b> in contact with the conveyor belt <b>31</b> and a discharging brush <b>193</b> for discharging the surface of the conveyor belt <b>31</b> are provided between the driven roller <b>33</b> and the charging roller <b>34</b>.

Further, a code wheel <b>137</b> is attached to a shaft <b>32</b><i>a </i>of the conveying roller <b>32</b> and an encoder sensor <b>138</b> implemented by a transmissive photosensor is provided to detect slits <b>137</b><i>a </i>formed in the code wheel <b>137</b>. The code wheel <b>137</b> and the encoder sensor <b>138</b> constitute a rotary encoder.

The paper feeding unit <b>4</b> includes a paper feed tray <b>41</b> that is removable from the body <b>1</b> and holds the paper sheets <b>5</b>; a paper feed roller <b>42</b> and a friction pad <b>43</b> for separating and feeding the paper sheets <b>5</b> one by one from the paper feed tray <b>41</b>; and resist rollers <b>44</b> for feeding the paper sheets <b>5</b> further to the paper conveying unit <b>3</b>.

The paper feeding unit <b>4</b> also includes a manual feed tray <b>46</b> for holding the paper sheets <b>5</b>, a manual feed roller <b>47</b> for feeding the paper sheets <b>5</b> one by one from the manual feed tray <b>46</b>, and vertical feed rollers <b>48</b> for feeding the paper sheets <b>5</b> fed from an optional paper feed tray attachable to the underside of the body <b>1</b> or from a duplex unit. The paper feed roller <b>42</b>, the resist rollers <b>44</b>, the manual feed roller <b>47</b>, and the vertical feed rollers <b>48</b>, which are used to feed the paper sheets <b>5</b> to the paper conveying unit <b>3</b>, are rotated by a paper feed motor (driving unit) <b>49</b>, implemented by an HB stepping motor, via an electromagnetic clutch (not shown).

The paper ejecting unit <b>6</b> includes paper ejecting rollers <b>61</b>, <b>62</b>, and <b>63</b> for conveying the paper sheet <b>5</b> on which an image has been formed, and paper ejecting rollers <b>64</b> and <b>65</b> for ejecting the paper sheet <b>5</b> to the paper catch tray <b>7</b>.

A control unit <b>300</b> of the image forming apparatus <b>200</b> is described below with reference to a block diagram shown in <figref idrefs="DRAWINGS">FIG. 5</figref>.

The control unit <b>300</b> includes a main control unit <b>310</b> comprising a CPU <b>301</b>, a ROM <b>302</b> for storing programs to be executed by the CPU <b>301</b> and other fixed data, a RAM <b>303</b> for temporarily storing image data, a non-volatile memory (NVRAM) <b>304</b> that can retain data even when the power is cut off, and an ASIC <b>305</b> that performs, for example, signal processing and sort operations on image data and handles input/output signals for controlling the image forming apparatus <b>200</b>. The main control unit <b>310</b> controls the entire image forming apparatus <b>200</b> and also controls processes of detecting and adjusting landing positions of liquid droplets.

The control unit <b>300</b> also includes an external I/F <b>311</b> for sending and receiving data and signals between the main control unit <b>310</b> and a host; a head control unit <b>312</b> including a head driver (disposed near the recording heads <b>24</b>) comprising a head data arrangement conversion ASIC for controlling the recording heads <b>24</b>; a main-scanning motor driving unit (motor driver) <b>313</b> for driving the main-scanning motor <b>27</b> that moves the carriage <b>23</b>; a sub-scanning motor driving unit (motor driver) <b>314</b> for driving the sub-scanning motor <b>131</b>; a paper feed motor driving unit <b>315</b> for driving the paper feed motor <b>49</b>; a paper ejecting motor driving unit <b>316</b> for driving a paper ejecting motor <b>79</b> that drives rollers in the paper ejecting unit <b>6</b>; an AC bias applying unit <b>319</b> for applying an AC bias voltage to the charging roller <b>34</b>; and a scanner control unit <b>325</b> for controlling the image scanning unit <b>11</b>. Although not shown in <figref idrefs="DRAWINGS">FIG. 5</figref>, the control unit <b>300</b> further includes a maintenance/cleaning motor driving unit for driving a maintenance/cleaning motor that drives the maintenance/cleaning mechanism <b>121</b>; a duplex unit driving unit for driving a duplex unit; a solenoid driving unit (driver) for driving solenoids (SOLs); and a clutch driving unit for driving electromagnetic clutches.

The main control unit <b>310</b> receives a detection signal from an environmental sensor <b>234</b> that detects the temperature and humidity (environmental conditions) around the conveyor belt <b>31</b>. Although the main control unit <b>310</b> also receives detection signals from other sensors, those sensors are omitted in <figref idrefs="DRAWINGS">FIG. 5</figref>. The main control unit <b>310</b> receives key inputs from and sends display information to an operations/display unit <b>327</b> on the body <b>1</b>. The operations/display unit <b>327</b> includes keys, such as numeric keys and a print start key, and displays.

Also, the main control unit <b>310</b> receives a signal from the encoder sensor <b>129</b> constituting a part of the linear encoder for detecting the position of the carriage <b>23</b>. Based on the received signal, the main control unit <b>310</b> causes the main-scanning motor driving unit <b>313</b> to drive the main-scanning motor <b>27</b> and thereby moves the carriage <b>23</b> back and forth in the main-scanning direction. Also, the main control unit <b>310</b> receives a signal (pulse) from the encoder sensor <b>138</b> constituting a part of the rotary encoder for detecting the amount of movement of the conveyor belt <b>31</b>. Based on the received signal, the main control unit <b>310</b> causes the sub-scanning motor driving unit <b>314</b> to drive the sub-scanning motor <b>131</b> to rotate the conveying roller <b>32</b> and thereby turns the conveyor belt <b>31</b>.

Further, the main control unit <b>310</b> causes the light-emitting element of the image sensor <b>401</b>, which scans a test pattern formed on the conveyor belt <b>31</b>, to emit light, detects the amount of positional deviation of liquid droplets based on a detection signal from the light-receiving element of the image sensor <b>401</b>, and adjusts the timing (liquid-jet timing) of jetting liquid droplets from the recording heads <b>24</b> based on the detected amount of positional deviation. Details of this process are described later.

An exemplary image forming process in the image forming apparatus <b>200</b> is described below. The main control unit <b>310</b> detects the amount of rotation of the conveying roller <b>32</b> that drives the conveyor belt <b>31</b>, and controls the sub-scanning motor <b>131</b> based on the detected amount of rotation. Meanwhile, the main control unit <b>310</b> causes the AC bias applying unit <b>319</b> to apply a high AC voltage having a rectangular wave with positive and negative peaks to the charging roller <b>34</b>. The charging roller <b>34</b> charges the conveyor belt <b>31</b> and forms positively-charged and negatively-charged strip-shaped-areas alternately in the paper conveying direction. As a result, a non-uniform electric field is formed on the conveyor belt <b>31</b>.

The paper sheet <b>5</b> is fed from the paper feeding unit <b>4</b> into the space between the conveying roller <b>32</b> and the first pressing roller <b>36</b>, and is placed on the conveyor belt <b>31</b> where the non-uniform electric field is formed. When placed on the conveyor belt <b>31</b>, the paper sheet <b>5</b> is instantly polarized along the direction of the electric field, thereby electrostatically attracted to the conveyor belt <b>31</b>, and conveyed as the conveyor belt <b>31</b> turns.

The paper sheet <b>5</b> is intermittently conveyed by the conveyor belt <b>31</b>. While the paper sheet <b>5</b> is momentarily stopped, the carriage <b>23</b> moves in the main-scanning direction and the recording heads <b>24</b> jet droplets of recording liquids onto the paper sheet <b>5</b> to form an image. Then, the paper sheet <b>5</b> is separated by the separating claws <b>39</b> from the conveyor belt <b>31</b>, fed into the paper ejecting unit <b>6</b>, and ejected onto the paper catch tray <b>7</b>.

When the image forming apparatus <b>200</b> is in a standby mode, the carriage <b>23</b> is moved into a position above the maintenance/cleaning mechanism <b>121</b>. In the position, the nozzle surfaces <b>24</b><i>a </i>of the recording heads <b>24</b> are covered by the caps <b>122</b> to retain moisture in the nozzles and thereby to prevent nozzle clogging caused by dried ink. The moisture-retention/suction cap <b>122</b><i>a </i>also suctions the nozzles of any one of the recording heads <b>24</b> being covered to remove dried ink or air bubbles. Ink adhered to the nozzle surfaces <b>24</b><i>a </i>of the recording heads <b>24</b> during this cleaning process is wiped off by the wiper blade <b>124</b>. Also, before or during an image forming process, ink is jetted into the waste-ink receiver <b>125</b> in order to clean the nozzles. With the above measures, performance of the recording heads <b>24</b> is maintained.

A first embodiment of the present invention is described below. First, a mechanism for detecting and adjusting landing positions of liquid droplets in the image forming apparatus <b>200</b> is described with reference to <figref idrefs="DRAWINGS">FIGS. 6 and 7</figref>. <figref idrefs="DRAWINGS">FIG. 6</figref> is a block diagram illustrating an exemplary mechanism for detecting and adjusting landing positions of liquid droplets. FIG. <b>7</b> is a drawing illustrating the exemplary mechanism for detecting and adjusting landing positions of liquid droplets in more detail.

As shown in <figref idrefs="DRAWINGS">FIG. 7</figref> (see <figref idrefs="DRAWINGS">FIG. 9</figref> also), the carriage <b>23</b> is equipped with the image sensor <b>401</b> (detecting unit) that detects a test pattern <b>400</b> (may also be called an adjustment pattern or a detection pattern) formed on the conveyor belt <b>31</b> made of a water-repellent material. The image sensor <b>401</b> includes a light-emitting element <b>402</b> for illuminating the test pattern <b>400</b> on the conveyor belt <b>31</b> and a light-receiving element <b>403</b> for receiving specularly reflected light from the test pattern <b>400</b>. Actually, the light-emitting element <b>402</b> also illuminates the surface of the conveyor belt <b>31</b> and the light-receiving element <b>403</b> also receives specularly reflected light from the surface of the conveyor belt. The light-emitting element <b>402</b> and the light-receiving element <b>403</b> are held in a holder <b>404</b>. A lens <b>405</b> is provided at a light exit/entry opening of the holder <b>404</b>.

As shown in <figref idrefs="DRAWINGS">FIG. 2</figref>, in the holder <b>404</b>, the light-emitting element <b>402</b> and the light-receiving element <b>403</b> are arranged in a direction orthogonal to the main-scanning direction of the carriage <b>23</b>. This arrangement reduces the influence of variation in the moving speed of the carriage <b>23</b> on detection results of the image sensor <b>401</b>. As the light-emitting element <b>402</b>, a comparatively simple and inexpensive light source, such as a LED, that emits infrared light or visible light may be used. The spot diameter (detection range or detection area) of a light source is preferably on the order of millimeters to allow the use of an inexpensive lens instead of an expensive, high-precision lens.

When requested to perform a landing position adjusting process, a test pattern formation/scanning control unit <b>501</b> (may also be called a pattern formation control unit) requests a liquid-jetting control unit <b>502</b> to jet liquid droplets from the recording heads <b>24</b> onto the conveyor belt <b>31</b> while moving the carriage <b>23</b> back and forth in the main-scanning direction, and thereby forms test patterns <b>400</b> (<b>400</b>B<b>1</b>, <b>400</b>B<b>2</b>, <b>400</b>C<b>1</b>, and <b>400</b>C<b>2</b>) composed of separate liquid droplets <b>500</b> as shown in <figref idrefs="DRAWINGS">FIG. 8</figref>. The test pattern formation/scanning control unit <b>501</b> may be implemented by the CPU <b>301</b> of the main control unit <b>310</b>.

The test pattern formation/scanning control unit <b>501</b> also controls a process of scanning the test patterns <b>400</b> with the image sensor <b>401</b>. In this process, the test pattern formation/scanning control unit <b>501</b> causes the light-emitting element <b>402</b> of the image sensor <b>401</b> to emit light while moving the carriage <b>23</b> in the main-scanning direction. More specifically, as shown inn <figref idrefs="DRAWINGS">FIG. 7</figref>, the CPU <b>301</b> of the main control unit <b>310</b> sets a PWM value, based on which the light-emitting element <b>402</b> of the image sensor <b>401</b> is driven, in a light-emission control unit <b>511</b>. A smoothing circuit <b>512</b> smoothes an output signal from the light-emission control unit <b>511</b> and outputs the smoothed signal to a drive circuit <b>513</b>. The drive circuit <b>513</b> causes the light-emitting element <b>402</b> to illuminate each of the test patterns <b>400</b> on the conveyor belt <b>31</b>.

Specularly reflected light from the test pattern <b>400</b> illuminated by the light-emitting element <b>402</b> enters the light-receiving element <b>403</b> of the image sensor <b>401</b>. The light-receiving element <b>403</b> outputs a detection signal proportional to the amount of received specularly reflected light to a positional deviation calculation unit <b>503</b> of a landing position adjusting unit <b>505</b>. More specifically, as shown in <figref idrefs="DRAWINGS">FIG. 7</figref>, a photoelectric conversion circuit <b>521</b> (not shown in <figref idrefs="DRAWINGS">FIG. 5</figref>) of the main control unit <b>310</b> photoelectrically converts the detection signal from the light-receiving element <b>403</b>. A low-pass filter <b>522</b> removes noise from the photoelectrically converted signal (sensor output voltage). An A/D converter <b>523</b> converts the sensor output voltage from analog to digital, and a digital signal processor (DSP) <b>524</b> stores the converted sensor output voltage in a common memory <b>525</b>.

The positional deviation calculation unit <b>503</b> of the landing position adjusting unit <b>505</b> determines the position of the test pattern <b>400</b> (or the position of each line pattern constituting the test pattern <b>400</b>) based on the detection signal from the light-receiving element <b>403</b> and calculates positional deviation of liquid droplets from a reference position. The positional deviation calculated by the positional deviation calculation unit <b>503</b> is output to a liquid-jet-timing adjustment value calculation unit <b>504</b>. The liquid-jet-timing adjustment value calculation unit <b>504</b> calculates an adjustment value for adjusting a liquid-jet timing at which the recording head <b>24</b> is driven, and sets the adjustment value in the liquid-jetting control unit <b>502</b>. The liquid-jetting control unit <b>502</b> adjusts the liquid-jet timing based on the adjustment value and drives the recording head <b>24</b> at the adjusted liquid-jet timing so as to reduce the positional deviation of liquid droplets.

The above process in the landing position adjusting unit <b>505</b> is described in more detail below with reference to <figref idrefs="DRAWINGS">FIG. 7</figref>. The landing position adjusting unit <b>505</b> is implemented by a processing algorithm <b>526</b> executed by the CPU <b>301</b>. The processing algorithm <b>526</b> determines the center point (point A) of each of line patterns constituting the test pattern <b>400</b> (<b>400</b><i>a </i>indicates each of the line patterns; each of the line patterns may also be called a test pattern) based on a sensor output voltage So, calculates the positional deviation of liquid droplets jetted from the corresponding one of the recording heads <b>24</b> with respect to the reference position (reference head), calculates an adjustment value for adjusting the liquid-jet timing based on the positional deviation, and sets the adjustment value in the liquid-jetting control unit <b>502</b>.

The test pattern <b>400</b> according to an embodiment of the present invention is described below.

First, a mechanism of detecting landing positions of liquid droplets (a pattern) is described. <figref idrefs="DRAWINGS">FIG. 10</figref> is a drawing illustrating diffusely reflected light from a liquid droplet <b>500</b> (may also be called an ink droplet <b>500</b>).

As shown in <figref idrefs="DRAWINGS">FIG. 10</figref>, the liquid droplet <b>500</b> jetted onto a target surface <b>600</b> has a glossy hemispherical surface. Therefore, most of incident light <b>601</b> on the liquid droplet <b>500</b> is reflected as diffusely reflected light <b>602</b> and only a small portion of the incident light <b>601</b> is reflected as specularly reflected light <b>603</b>. However, as shown in <figref idrefs="DRAWINGS">FIG. 11</figref>, the liquid droplet <b>500</b> gradually flattens and its surface becomes less glossy as it dries over time. As a result, the proportion of the specularly reflected light <b>603</b> to the diffusely reflected light <b>602</b> increases. Therefore, as shown in <figref idrefs="DRAWINGS">FIG. 12</figref>, the sensor output voltage based on the specularly reflected light <b>603</b> received by the light-receiving element <b>403</b> increases and the detection accuracy decreases as time passes.

Next, an exemplary mechanism of detecting positions of the liquid droplets <b>500</b> forming the test patterns <b>400</b> is described with reference to <figref idrefs="DRAWINGS">FIG. 13</figref>.

The conveyor belt <b>31</b> has a glossy surface (belt surface) that reflects most of the light from the light-emitting element <b>401</b> as specularly reflected light. Therefore, the amount of specularly reflected light <b>603</b> from a droplet-absent area of the belt surface, where the liquid droplets <b>500</b> are not present, is large as shown in <figref idrefs="DRAWINGS">FIG. 13(</figref><i>b</i>), and the sensor output voltage output from the light-receiving element <b>403</b> when receiving the specularly reflected light <b>603</b> from the droplet-absent area becomes comparatively large as shown in <figref idrefs="DRAWINGS">FIG. 13(</figref><i>a</i>).

On the other hand, the amount of specularly reflected light <b>603</b> from a droplet-present area of the belt surface, where the liquid droplets <b>500</b> are present and separated from each other, is small as shown in <figref idrefs="DRAWINGS">FIG. 13(</figref><i>b</i>), and the sensor output voltage output from the light-receiving element <b>403</b> when receiving the specularly reflected light <b>603</b> from the droplet-present area becomes comparatively small as shown in <figref idrefs="DRAWINGS">FIG. 13(</figref><i>a</i>). Accordingly, it is possible to detect landing positions of ink droplets (or a test pattern) by the difference in the level of an output voltage from the light-receiving element <b>403</b>. In other words, it is possible to detect the test pattern <b>400</b> based on a low-level portion of the detection signal from the light-receiving element <b>403</b> which low-level portion indicates that the amount of specularly reflected light is small.

Meanwhile, if adjoining liquid droplets <b>500</b> clump together on the conveyor belt <b>31</b> as shown in <figref idrefs="DRAWINGS">FIG. 14(</figref><i>b</i>) to form a larger liquid droplet <b>500</b> with a flat top surface, the amount of specularly reflected light <b>603</b> from the droplet-present area increases, and the sensor output voltage of the droplet-present area becomes substantially as large as that of the liquid-absent area. This in turn makes it difficult to detect the position of the liquid droplet <b>500</b>. Although a small portion of the incident light is diffusely reflected at the edge of the liquid droplet <b>500</b> formed as a result of clumping, it is difficult to detect the diffusely reflected light since the diffuse reflection occurs in a very small area. Reducing the coverage area of the light-receiving element <b>403</b> (the area that the light-receiving element <b>403</b> can detect at once) may make it possible to detect such a very small area. However, reducing the coverage area increases noise in detection results caused by tiny foreign objects or flaws on the surface of the conveyor belt <b>31</b>, and therefore reduces the accuracy and reliability of the detection results.

To reduce or obviate the above problem, i.e., to accurately detect landing positions of ink droplets, the test pattern <b>400</b> is preferably composed of separate ink droplets in the detection range of the image sensor <b>401</b>. Using such a test pattern, in turn, makes it possible to accurately detect a test pattern (or landing positions of liquid droplets) with a simple image sensor including a light-emitting element and a light-receiving element. Also, separate liquid droplets forming the test pattern <b>400</b> are preferably arranged densely. In other words, in a detection range of the detecting unit, an area of the test pattern not occupied by the liquid droplets is preferably smaller than an area of the test pattern occupied by the liquid droplets.

A difference between a test pattern formed with toner and a test pattern formed with liquid droplets is described below with reference to <figref idrefs="DRAWINGS">FIG. 15</figref>.

Toner used in electrophotographic printing does not change its shape even after being transferred onto a target surface. Therefore, when a test pattern is formed on a target surface <b>610</b> with toner <b>611</b>, the amount of specularly reflected light from a toner-present area of the target surface <b>610</b> is constantly smaller than that from a toner-absent area of the target surface <b>610</b>. In other words, when a test pattern is formed with toner, it is possible to accurately detect the test pattern based on an output voltage from a light-receiving element for receiving specularly reflected light.

On the other hand, as described above, when a test pattern is formed with liquid droplets, the liquid droplets tend to clump together to form a larger liquid droplet with a flat top surface, and the amount of specularly reflected light from a droplet-present area becomes substantially the same as that from a droplet-absent area. Without taking into account such characteristics of liquid droplets, it is not possible to accurately detect a test pattern based on the amount of specularly reflected light. Embodiments of the present invention provide a liquid-jet device and an image forming apparatus that can form a test pattern composed of separate liquid droplets, accurately detect the test pattern based on the amount of specularly reflected light from the test pattern, and thereby accurately adjust landing positions of liquid droplets.

Exemplary processes of detecting the position of the test pattern <b>400</b> formed on the conveyor belt <b>31</b> are described below with reference to <figref idrefs="DRAWINGS">FIGS. 16A through 18</figref>.

<figref idrefs="DRAWINGS">FIGS. 16A and 16B</figref> are drawings used to describe a first exemplary position detecting process. In the first exemplary position detecting process, line patterns (test patterns) <b>400</b><i>k</i><b>1</b> and <b>400</b><i>k</i><b>2</b> are formed, respectively, by the recording heads <b>24</b><i>k</i><b>1</b> and <b>24</b><i>k</i><b>2</b> on the conveyor-belt <b>31</b> as shown in <figref idrefs="DRAWINGS">FIG. 16A</figref>. The line patterns <b>400</b><i>k</i><b>1</b> and <b>400</b><i>k</i><b>2</b> are scanned in the sensor-scanning direction (the main-scanning direction of the carriage <b>23</b>) by the image sensor <b>401</b>. As shown in <figref idrefs="DRAWINGS">FIG. 16B</figref>, the light-receiving element <b>403</b> of the image sensor <b>401</b> outputs a sensor output voltage So that falls at positions corresponding to the line patterns <b>400</b><i>k</i><b>1</b> and <b>400</b><i>k</i><b>2</b>.

Then, the sensor output voltage So is compared with a predetermined threshold value Vr, and positions at which the sensor output voltage So becomes lower than the threshold value Vr are detected as edges of the corresponding line patterns <b>400</b><i>k</i><b>1</b> and <b>400</b><i>k</i><b>2</b>. That is, it is possible to obtain a center point of a low-level portion of a detection signal from the light-receiving element <b>403</b> by comparing the detection signal with a predetermined threshold value and to use the obtained center point as an edge of a line pattern (or a test pattern) Also, centroids of hatched areas (in <figref idrefs="DRAWINGS">FIG. 16B</figref>) surrounded by a line indicating the threshold value Vr and a line indicating the sensor output voltage So may be obtained and used as the centers of the corresponding line patterns <b>400</b><i>k</i><b>1</b> and <b>400</b><i>k</i><b>2</b>. In other words, it is possible to obtain a centroid of a low-level portion of a detection signal from the light-receiving element <b>403</b> by comparing the detection signal with a predetermined threshold value and to use the obtained centroid as an edge of a line pattern (or a test pattern). Using a centroid makes it possible to reduce an error caused by small fluctuation of the sensor output voltage.

<figref idrefs="DRAWINGS">FIGS. 17A and 17B</figref> are graphs used to describe a second exemplary position detecting process. In the second exemplary position detecting process, a sensor output voltage So as shown in <figref idrefs="DRAWINGS">FIG. 17A</figref> is obtained by scanning the line patterns <b>400</b><i>k</i><b>1</b> and <b>400</b><i>k</i><b>2</b> used in the first exemplary position detecting process with the image sensor <b>401</b>. <figref idrefs="DRAWINGS">FIG. 17B</figref> is an enlarged view of a falling portion of the sensor output voltage So.

The falling portion of the sensor output voltage So is searched in a direction indicated by an arrow Q<b>1</b> shown in <figref idrefs="DRAWINGS">FIG. 17B</figref> to find a point P<b>2</b> where the sensor output voltage So becomes equal to a lower threshold Vrd, and the found point P<b>2</b> is stored in a memory. Next, the falling portion of the sensor output voltage So is searched from the point P<b>2</b> in a direction indicated by an arrow Q<b>2</b> to find a point P<b>1</b> where the sensor output voltage So becomes equal to an upper threshold Vru, and the found point P<b>1</b> is stored in a memory. Next, a regression line L<b>1</b> is obtained from the sensor output voltage So between the points P<b>1</b> and P<b>2</b>, and an intersection C<b>1</b> of the regression line L<b>1</b> and an median value Vrc between the upper and lower thresholds Vru and Vrd is obtained. Similarly, a regression line L<b>2</b> is obtained for the rising portion of the sensor output voltage So, and an intersection C<b>2</b> of the regression line L<b>2</b> and the median value Vrc between the upper and lower thresholds Vru and Vrd is obtained. Then, a center point between the intersections C<b>1</b> and C<b>2</b> is obtained by the formula (C<b>1</b>+C<b>2</b>)/2, and a center line C<b>12</b> is obtained from the center point. The center line C<b>12</b> or an intermediate position between the regression lines L<b>1</b> and L<b>2</b> can be used as an edge of a line pattern (or a test pattern).

<figref idrefs="DRAWINGS">FIG. 18</figref> is a drawing used to describe a third exemplary position detecting process. In the third exemplary position detecting process, a sensor output voltage So as shown in <figref idrefs="DRAWINGS">FIG. 18(</figref><i>b</i>) is obtained by scanning the line patterns <b>400</b><i>k</i><b>1</b> and <b>400</b><i>k</i><b>2</b>, which are formed by the recording heads <b>24</b><i>k</i><b>1</b> and <b>24</b><i>k</i><b>2</b> as in the first exemplary position detecting process, with the image sensor <b>401</b>.

The processing algorithm <b>526</b> described above removes harmonic noise from a detection signal of the image sensor <b>401</b> using an IIR filter, estimates the quality of the detection signal (determines whether there are incompleteness, instability, and redundancy in the detection signal), detects sloping portions of the detection signal near a threshold Vr, and thereby obtains a regression curve. Next, intersections a<b>1</b>, a<b>2</b>, b<b>1</b>, and b<b>2</b> between the regression curve and the threshold Vr are obtained (for example, with a position counter implemented by an application specific IC (ASIC)). Then, a center point A between the intersections a<b>1</b> and a<b>2</b> and a center point B between the intersections b<b>1</b> and b<b>2</b> are obtained, and a distance L between the intermediate points A and B is obtained. Accordingly, the distance L indicates the distance between the line patterns <b>400</b><i>k</i><b>1</b> and <b>400</b><i>k</i><b>2</b>.

A difference between the distance L and an optimum distance between the recording heads <b>24</b><i>k</i><b>1</b> and <b>24</b><i>k</i><b>2</b> is obtained by subtracting the distance L from the optimum distance. The difference indicates the amount of positional deviation of liquid droplets. Based on the obtained amount of positional deviation, an adjustment value for adjusting the timing (liquid-jet timing) of jetting liquid droplets from the recording heads <b>24</b><i>k</i><b>1</b> and <b>24</b><i>k</i><b>2</b> is obtained and set in the liquid-jetting control unit <b>502</b>. The liquid-jetting control unit <b>502</b> drives the recording heads <b>24</b><i>k</i><b>1</b> and <b>24</b><i>k</i><b>2</b> at the adjusted liquid-jet timing to adjust the landing positions of liquid droplets.

Next, exemplary arrangements of liquid droplets forming the test pattern <b>400</b> are described.

<figref idrefs="DRAWINGS">FIG. 19</figref> is a drawing illustrating a first exemplary arrangement of liquid droplets forming the test pattern <b>400</b> (or a line pattern <b>400</b><i>a</i>) where separate liquid droplets <b>500</b> are arranged in a grid.

<figref idrefs="DRAWINGS">FIGS. 20A and 20B</figref> are drawings illustrating second exemplary arrangements of liquid droplets. In <figref idrefs="DRAWINGS">FIG. 20A</figref>, a larger droplet (primary droplet) and a smaller droplet (secondary droplet) are combined to form a pear-shaped liquid droplet <b>500</b>A, and separate liquid droplets <b>500</b>A are arranged in a grid. In <figref idrefs="DRAWINGS">FIG. 20B</figref>, two droplets of substantially the same size are combined to form a droplet <b>500</b>B, and separate liquid droplets <b>500</b>B are arranged in a grid.

<figref idrefs="DRAWINGS">FIGS. 21A and 21B</figref> are drawings illustrating third exemplary arrangements of liquid droplets. In <figref idrefs="DRAWINGS">FIG. 21A</figref>, multiple droplets are arranged in a direction orthogonal to the sensor-scanning direction and combined to form a line-shaped droplet <b>500</b>C, and multiple line-shaped droplets <b>500</b>C are arranged in the sensor-scanning direction. In <figref idrefs="DRAWINGS">FIG. 21B</figref>, each droplet <b>500</b>D is shaped like the droplet <b>500</b>C with one or more missing parts (the lengths of the droplets <b>500</b>C and <b>500</b>D may be either the same or different), and multiple droplets <b>500</b>D are arranged in the scanning direction of the image sensor <b>401</b>.

Arrangements of liquid droplets preferable to accurately detect the landing positions are described below with reference to <figref idrefs="DRAWINGS">FIGS. 22A through 22C</figref>.

First, it is necessary to maintain the proportion of diffusely reflected light in reflected light from the test pattern <b>400</b>. In other words, it is necessary to jet the liquid droplets <b>500</b> onto the conveyor belt <b>31</b> (or a target surface) in such a manner that the proportion of diffusely reflected light from the test pattern <b>400</b> becomes constant as shown in the middle of <figref idrefs="DRAWINGS">FIG. 13</figref>. Maintaining the proportion of diffusely reflected light improves reproducibility of the sensor output voltage (or a detection signal) to be processed by the processing algorithm <b>526</b>, and thereby makes it possible to accurately detect the test pattern <b>400</b> (landing positions of liquid droplets) and to accurately adjust landing positions of liquid droplets.

To maintain the proportion of diffusely reflected light from the test pattern <b>400</b>, it is necessary to keep constant a diffuse reflection area, which is the total area of surfaces (diffuse reflection surfaces) that diffusely reflect light, of liquid droplets. In the example shown in <figref idrefs="DRAWINGS">FIG. 22A</figref>, separate liquid droplets <b>500</b> forming the test pattern <b>400</b> are placed in every other dot position. With this arrangement, regularly-arranged (or regularly-spaced) liquid droplets do not clump together, and therefore the diffuse reflection area of the liquid droplets is kept constant. Also, as long as adjacent liquid droplets <b>500</b> are separated from each other, the liquid droplets <b>500</b> may be arranged in a staggered manner as shown in <figref idrefs="DRAWINGS">FIG. 22B</figref> or placed in all dot positions as shown in <figref idrefs="DRAWINGS">FIG. 22C</figref>.

As described above with reference to <figref idrefs="DRAWINGS">FIG. 12</figref>, liquid droplets dry over time after they are placed on a target surface and the proportion of diffusely reflected light from the liquid droplets changes. Therefore, to improve reproducibility of the sensor output voltage, it is preferable to cause the image sensor <b>401</b> to detect specularly reflected light at a predetermined timing after liquid droplets are placed on a target surface.

Further, as long as the proportion of diffusely reflected light is maintained, the test pattern <b>400</b> may be composed of regularly-arranged (or regularly-spaced) liquid droplets <b>500</b> each formed by two or more liquid droplets as shown in <figref idrefs="DRAWINGS">FIGS. 20A through 21B</figref>.

Meanwhile, to maintain the proportion of diffusely reflected light from the test pattern <b>400</b>, it is also preferable to keep constant the contact area of the liquid droplets <b>500</b> with the conveyor belt <b>31</b> in a detection range (detection area) <b>450</b> of the image sensor <b>401</b>. In the example shown in <figref idrefs="DRAWINGS">FIG. 23</figref>, separate liquid droplets <b>500</b> forming the test pattern <b>400</b> are placed in every other dot position. In this case, it is possible to make constant the contact area of the liquid droplets <b>500</b> with the conveyor belt <b>31</b> by jetting the same amount of liquid to form each of the liquid droplets <b>500</b>. As long as the liquid droplets <b>500</b> are separated from each other, the liquid droplets <b>500</b> may be arranged in a staggered manner. Also, using a pigment ink in combination with the conveyor belt <b>31</b> made of a fluorine resin (e.g., ethylene-tetrafluoroethylene (ETFE)), which is repellent to a pigment ink, makes it easier to keep the contact area constant.

Accordingly, it is possible to more effectively maintain the proportion of diffusely reflected light from a test pattern and improve the reproducibility of a sensor output voltage by maintaining the diffuse reflection area of liquid droplets and by keeping constant the contact area of liquid droplets with a conveyor belt at the same time.

It is also important to arrange liquid droplets sufficiently densely to obtain a detection result high enough to determine whether the test pattern <b>400</b> is present. <figref idrefs="DRAWINGS">FIG. 24</figref> is a graph showing the relationship obtained by an experiment between the proportion of the diffuse reflection area of liquid droplets in the total area of the test pattern <b>400</b> and a detection result. As shown in <figref idrefs="DRAWINGS">FIG. 24</figref>, a sufficient detection result can be obtained when the proportion of the diffuse reflection area in the total area of the test pattern <b>400</b> is 10% or larger.

Next, characteristics of liquid droplets forming the test pattern <b>400</b> are described in terms of a pattern diffuse reflection rate.

A pattern diffuse reflection rate indicates the proportion of the diffuse reflection area in the detection range (see <figref idrefs="DRAWINGS">FIG. 23</figref>) of the image sensor <b>401</b>. A pattern diffuse reflection rate is obtained by the following formula: pattern diffuse reflection rate=diffuse reflection area/detection range area (*detection range area indicates the area of a surface that can be covered by the image sensor <b>401</b> at once).

Assuming that the detection range of the image sensor <b>401</b> is constant, the pattern diffuse reflection rate can be increased by increasing the diffuse reflection area. As shown in <figref idrefs="DRAWINGS">FIG. 25</figref>, when the wettability of the surface of the conveyor belt <b>31</b> is low (when the surface has a large contact angle θ (see <figref idrefs="DRAWINGS">FIG. 26</figref>) with the liquid droplet <b>500</b>), the liquid droplet <b>500</b> on the conveyor belt <b>31</b> takes on a hemispherical shape. In this case, a portion <b>500</b><i>a </i>of the outer surface of the liquid droplet <b>500</b> specularly reflects light arriving from a given direction, and a portion <b>500</b><i>b </i>diffusely reflects the light. The diffuse reflection area of each liquid droplet <b>500</b> or a droplet diffuse reflection rate can be increased by jetting the liquid droplet <b>500</b> in such a manner that the portion <b>500</b><i>b </i>becomes large.

The droplet diffuse reflection rate indicates the proportion of the diffuse reflection area (portion <b>500</b><i>b</i>) of a liquid droplet with respect to the contact area of the liquid droplet with the conveyor belt <b>31</b>, and can be obtained by the following formula: droplet diffuse reflection rate=diffuse reflection area of droplet/contact area.

Also, it is preferable to use the largest liquid droplets (with the largest droplet volume) available for image formation (or the largest liquid droplets that the recording heads <b>24</b> can jet) to form the test pattern <b>400</b>. In other words, it is preferable to form the test pattern <b>400</b> in a print mode that uses largest liquid droplets. Using the largest liquid droplet makes it possible to increase the height of the liquid droplet <b>500</b> shown in <figref idrefs="DRAWINGS">FIG. 25</figref> and thereby to increase the droplet diffuse reflection rate.

Meanwhile, the composition of ink varies depending on its color (e.g., cyan, magenta, yellow, or black), and the shape of the liquid (ink) droplet <b>500</b> may vary depending on the composition of ink used. Therefore, to effectively increase the droplet diffuse reflection rate, it is more preferable to change the size (or volume) of ink droplets used to form the test pattern <b>400</b> depending on the color of the ink.

In a landing position adjusting process described above, a test pattern is formed on a conveyor belt by jetting liquid droplets with a liquid-jetting unit, the test pattern is illuminated by a light-emitting element, specularly reflected light from the test pattern is received by a light-receiving element, and landing positions of the liquid droplets are adjusted based on a low-level portion of a detection signal from the light-receiving element. In this process, it is possible to improve the detection results of the light-receiving element (a sensor) by controlling the liquid-jetting unit so as to maximize the pattern diffuse reflection rate of liquid droplets forming the test pattern and thereby to accurately detect and adjust the landing positions of the liquid droplets.

The pattern diffuse reflection rate can be increased by controlling the liquid-jetting unit so as to increase the diffuse reflection area of each liquid droplet or the droplet diffuse reflection rate. The droplet diffuse reflection rate can be increased, for example, by

(1) Controlling the amount of liquid jetted to form a liquid droplet (or controlling the volume of a liquid droplet);

(2) Controlling the amount of liquid jetted to form a liquid droplet depending on the color of the liquid;

(3) Reducing the time lag between formation of a test pattern (or jetting liquid droplets) and scanning of the test pattern by light-emitting and light-receiving elements, and performing the formation and scanning of the test pattern at substantially the same time in one operation;

(4) Selecting materials of a conveyor belt and a liquid (or ink) such that the conveyor belt has a large contact angle with a liquid droplet;

(5) Using liquid droplets that take on a circular shape or a pear shape on a conveyor belt; and

(6) Maximizing the area occupied by substantially separate liquid droplets in the detection range of an image sensor (light-emitting and light-receiving elements) by, for example, arranging liquid droplets such that the gaps between them are minimized.

Next, a method of forming and detecting the test pattern <b>400</b> is described. As described above, the shape of a liquid droplet changes because it dries over time after it is jetted onto the belt surface. Therefore, the proportion of specularly reflected light from the test pattern <b>400</b> increases as time passes and the sensor output voltage from the image sensor <b>401</b> increases.

To obviate this problem and to accurately detect landing positions of liquid droplets, it is preferable to scan the test pattern <b>400</b> with the image sensor <b>401</b> just after the test pattern <b>400</b> is formed. This objective can be achieved, for example, by forming the test pattern <b>400</b> at a test-pattern forming speed and scanning the test pattern <b>400</b> as it is formed at a scanning speed that is substantially the same as the test-pattern forming speed. In this case, it is necessary to dispose the image sensor <b>401</b> upstream of the carriage <b>23</b> with respect to the direction in which the carriage <b>23</b> is moved to form the test pattern <b>400</b>. With this configuration, the test pattern <b>400</b> must be formed by moving the carriage <b>23</b> in one direction only (either by the forward scan or the backward scan).

The above objective can also be achieved with a configuration where the test pattern <b>400</b> is formed at a test-pattern forming speed by both the forward and backward scans of the carriage <b>23</b> and is scanned by the image sensor <b>401</b> at a scanning speed different from the test-pattern forming speed without turning the conveyor belt <b>31</b>. In this case, it is necessary to dispose the image sensor <b>401</b> so that it is positioned in an area where the test pattern <b>400</b> is formed.

Exemplary composition of pigment inks that can increase the droplet diffuse reflection rate when used in combination with the conveyor belt <b>31</b> made of a fluorine resin (e.g., ethylene-tetrafluoroethylene (ETFE)) is described below. For example, pigment inks containing materials as described below are preferably used.

Examples of preferable organic pigments include azo series, phthalocyanine series, anthraquinone series, quinacridone series, dioxazine series, indigo series, thioindigo series, perylene series, isoindolinon series, aniline black, azomethine series, rhodamine B lake pigment, and carbon black.

Examples of preferable inorganic pigments include iron oxide, titanium oxide, calcium carbonate, barium sulfate, aluminum hydroxide, barium yellow, iron blue, cadmium red, chrome-yellow, and metallic flake.

The particle diameter of a pigment is preferably between 0.01 and 0.30 μm. If the particle diameter is smaller than 0.01 μm and is close to that of dye particles, the pigment shows low light resistance and causes feathering. If the particle diameter is larger than 0.30 μm, the pigment particles may clog nozzles or filters in an image forming apparatus and thereby reduces ink-jetting performance.

Preferably, carbon black for a black pigment ink is made by a furnace method or a channel method, and has a primary particle diameter of 15-40 nm (millimicrons), a BET specific surface area of 50-300 m²/g, a DBP oil absorption of 40-150 ml/100 g, a volatile matter content of 0.5-10%, and a pH value of 2-9. Examples of preferable carbon blacks include No. 2300, No. 900, MCF-88, No. 33, No. 40, No. 45, No. 52, MA7, MA8, MA100, No. 2200B (Mitsubishi Chemical Corporation); Raven 700, Raven 5750, Raven 5250, Raven 5000, Raven 3500, Raven 1255 (Columbian Chemicals Company); Regal 400R, Regal 330R, Regal 660R, MogulL, Monarch 700, Monarch 800, Monarch 880, Monarch 900, Monarch 1000, Monarch 1100, Monarch 1300, Monarch 1400 (Cabot Corporation); Color black FW1, Color black FW2, Color black FW2V, Color black FW18, Color black FW200, Color black S150, Color black S160, Color black S170, Printex 35, Printex U, Printex V, Printex 140U, Printex 140V, Special black 6, Special black 5, Special black 4A, and Special black 4 (Degussa).

Examples of preferable color pigments are listed below.

Examples of color organic pigments include azo series, phthalocyanine series, anthraquinone series, quinacridone series, dioxazine series, indigo series, thioindigo series, perylene series, isoindolinon series, aniline black, azomethine series, rhodamine B lake pigment, and carbon black. Examples of color inorganic pigments include iron oxide, titanium oxide, calcium carbonate, barium sulfate, aluminum hydroxide, barium yellow, iron blue, cadmium red, chrome yellow, and metallic flake.

More specifically, pigments as described below may be used for each color.

The following pigments may be used for yellow ink: CI pigment yellow 1, 2, 3, 12, 13, 14, 16, 17, 73, 74, 75, 83, 93, 95, 97, 98, 114, 128, 129, 151, and 154.

The following pigments may be used for magenta ink: CI pigment red 5, 7, 12, 48 (Ca), 48 (Mn), 57 (Ca), 57:1, 112, 123, 168, 184, and 202.

The following pigments may be used for cyan ink: CI pigment blue 1, 2, 3, 15:3, 15:34, 16, 22, and 60; and CI vat blue 4 and 60.

An inkjet recording liquid may be prepared by dispersing one of the above pigments in an aqueous medium using a polymer dispersant or a surfactant. As a dispersant for dispersing organic pigment powder, a water-soluble resin or a water-soluble surfactant may be used.

Examples of preferable water-soluble resins include a block copolymer, a random copolymer, and salt composed of two or more monomers selected from a group including styrene, styrene derivative, vinylnaphthalene derivative, aliphatic alcohol ester of α,β-ethylene unsaturated carboxylic acid, acrylic acid, acrylic acid derivative, maleic acid, maleic acid derivative, itaconic acid, itaconic acid derivative, fumarate, and fumarate derivative. The above water-soluble resins are alkali-soluble resins that are soluble in water solution of a base. A water-soluble resin with a weight-average molecular weight of 3000-20000 is easily dispersible, is suitable to prepare a dispersion liquid with a low viscosity, and is therefore especially preferable for an inkjet recording liquid.

As a water-soluble surfactant, an anionic surfactant, a cationic surfactant, an amphoteric surfactant, or a nonionic surfactant may be used. Examples of anionic surfactants include higher fatty acid salt, alkyl sulfate, alkyl ether sulfate, alkyl ester sulfate, alkyl arylether sulfate, alkyl sulfonate, sulfosuccinate, alkyl allyl and alkyl naphthalene sulfonate, alkyl phosphate, polyoxyethylene alkyl ether phosphate ester salt, and alkyl allyl ether phosphate. Examples of cationic surfactants include alkyl amine salt, dialkyl amine salt, tetraalkyl ammonium salt, benzalkonium salt, alkyl pyridinium salt, and imidazolinium salt. Examples of amphoteric surfactants include dimethyl alkyl lauryl betaine, alkyl glycine, alkyldi (aminoethyl) glycine, and imidazolinium betaine. Examples of nonionic surfactants include polyoxyethylene alkyl ether, polyoxyethylene alkyl allyl ether, polyoxyethylene polyoxypropylene glycol, glycerin ester, sorbitan ester, sucrose esters, polyoxyethylene ether of glycerin ester, polyoxyethylene ether of sorbitan ester, polyoxyethylene ether of sorbitol ester, fatty acid alkanolamide, polyoxyethylene fatty acid amide, amine oxide, and polyoxyethylene alkylamine.

A pigment may be microencapsulated by coating it with a resin having a hydrophilic radical. Microencapsulating gives the pigment dispersibility.

Any of conventional methods may be used to microencapsulate a water-insoluble pigment by coating it with an organic polymer. Such conventional methods include chemical manufacturing methods, physical manufacturing methods, physicochemical manufacturing methods, and mechanical manufacturing methods. For example, microencapsulation methods (1) through (10) described below may be used.

(1) Interface polymerization method: two types of monomers or two types of reactants are dissolved in a disperse phase and a continuous phase separately, and are caused to react with each other at the interface between the two phases and thereby to form wall membranes.

(2) In-situ polymerization method: aqueous or gaseous monomers and catalysts or two types of reactive substances are supplied from either the continuous phase side or the nuclear particle side, and are caused to react with each other and thereby to form wall membranes.

(3) In-liquid curing coating method: wall membranes are formed by insolubilizing drops of polymer solution containing core material particles in a liquid using a curing agent.

(4) Coacervation (phase separation) method: wall membranes are formed by separating a polymer dispersed liquid, where core material particles are dispersed, into coacervate (dense phase) with a high polymer concentration and a dilute phase.

(5) In-liquid drying method: a core material is dispersed in a solution of a wall membrane material, the core material dispersed liquid is put in another liquid, in which the continuous phase of the core material dispersed liquid do not blend, to form a multiple emulsion, then the medium in which the wall membrane material is dissolved is gradually removed to form wall membranes.

(6) Melting-dispersion-cooling process: a wall membrane material that melts when heated and solidifies at normal temperature is liquefied by heating, core material particles are dispersed in the resulting liquid, and then the liquid is changed into fine particles and cooled to form wall membranes.

(7) In-air suspension coating method: powder of core material particles is suspended in air using a fluid bed, and a coating liquid used as a wall membrane material is sprayed in the air to form wall membranes.

(8) Spray drying method: an undiluted encapsulation liquid is sprayed and brought into contact with heated air to evaporate its volatile matter content and thereby to form wall membranes.

(9) Acidification deposition method: an organic polymer, at least a part of the anionic groups of which is neutralized with a basic compound to give it water solubility, is kneaded together with a colorant in an aqueous medium, neutralized or acidified using an acidic compound so that the organic polymer is deposited and fixed to the colorant, and then neutralized again and dispersed.

(10) Phase inversion emulsification: water is put in an organic solvent phase made of a mixture of a colorant and an anionic organic polymer having water dispersibility, or the organic solvent phase is put in water.

As a material for the wall membrane of a microcapsule, the following organic polymers (resins) may be used: polyamide, polyurethane, polyester, polyurea, epoxy resin, polycarbonate, urea resin, melamine resin, phenolic resin, polysaccharide, gelatin, acacia gum, dextran, casein, protein, natural rubber, carboxypolymethylene, polyvinyl alcohol, polyvinyl pyrrolidone, polyvinyl acetate, polyvinyl chloride, polyvinylidene chloride, cellulose, ethyl cellulose, methyl cellulose, cellulose nitrate, hydroxyethyl cellulose, cellulose acetate, polyethylene, polystyrene, polymer or copolymer of (metha)acrylic acid, polymer or copolymer of (metha)acrylic acid ester, (metha)acrylic acid-(metha)acrylic acid ester copolymer, styrene-(metha)acrylic acid copolymer, styrene-maleic acid copolymer, alginic acid soda, fatty acid, paraffin, bees wax, water wax, hardened tallow, carnauba wax, and albumin.

Among them, organic polymers having an anionic group such as a carboxylic group or a sulfonic group are preferable. Also, nonionic organic polymers such as polyvinyl alcohol, polyethylene glycol monomethacrylate, polypropylene glycol monomethacrylate, methoxypolyethylene glycol monomethacrylate, (co)polymers of the preceding substances, and cationic ring-opening polymer of 2-oxazoline may be used. Especially, completely saponified polyvinyl alcohol is preferable because of its low water solubility (it is easily soluble in hot water but not in cold water).

The amount of an organic polymer in a wall membrane material for microencapsulation is preferably 1-20 weight percent of a water-insoluble colorant such as an organic pigment or carbon black. Keeping the amount of organic polymer within the above range prevents the organic polymer coating the surface of a pigment from inhibiting the color development of the pigment. When the amount of an organic polymer is less than 1 weight percent, the effect of encapsulation becomes insufficient. When the amount of an organic polymer is more than 20 weight percent, the color development of a pigment is inhibited greatly. With other factors also taken into account, the amount of an organic polymer is more preferably 5-10 weight percent of a water-insoluble colorant.

With the amount of an organic polymer kept within the above range, a part of a colorant is substantially left uncoated or exposed, and therefore the color development of the colorant is not inhibited. From a different point of view, the colorant is not exposed but substantially coated, and therefore a sufficient encapsulation effect can be obtained. The number average molecular weight of an organic polymer is preferably 2,000 or more to efficiently perform encapsulation. “Substantially left uncoated or exposed” in this case means that a part of a colorant is intentionally left uncoated and does not include cases where a part of a colorant is exposed because of a defect such as a pinhole or a crack in the coating.

Using a self-dispersing organic pigment or a self-dispersing carbon black as a colorant gives high dispersibility to a microencapsulated pigment even when the content of an organic polymer in the capsule is relatively low. Therefore, a self-dispersing organic pigment and a self-dispersing carbon black are preferable as colorants to give sufficient preservation stability to an ink.

Also, it is preferable to select an appropriate organic polymer depending on the method of microencapsulation. For the interface polymerization method, for example, polyester, polyamide, polyurethane, polyvinyl pyrrolidone, and epoxy resin are preferable. For the in-situ polymerization method, polymer or copolymer of (metha)acrylic acid ester, (metha)acrylic acid-(metha)acrylic acid ester copolymer, styrene-(metha)acrylic acid copolymer, polyvinyl chloride, polyvinylidene chloride, and polyamide are preferable. For the in-liquid curing coating method, alginic acid soda, polyvinyl alcohol, gelatin, albumin, and epoxy resin are preferable. For the coacervation method, gelatin, cellulose, and casein are preferable. Other microencapsulation methods may also be used to obtain a fine, uniform microencapsulated pigment.

For the phase inversion emulsification method and the acidification deposition method, anionic organic polymers may be used. In the phase inversion emulsification method, one of the following is preferably used as an organic solvent phase: a mixture of an anionic organic polymer having self-dispersibility or solubility in water and a colorant such as a self-dispersing organic pigment or a self-dispersing carbon black; and a mixture of a colorant such as a self-dispersing organic pigment or a self-dispersing carbon black, a curing agent, and an anionic organic polymer. In this method, water is put in the organic solvent phase or the organic solvent phase is put in water. The organic solvent phase self-disperses (inversion emulsification) and the colorant is microencapsulated. In the phase inversion emulsification method, a recording liquid vehicle or additives may also be mixed in the organic solvent phase. Especially, mixing a recording liquid medium is preferable since it makes it possible to directly produce a dispersion liquid for a recording liquid.

In the acidification deposition method, a part or all of the anionic groups of an organic polymer are neutralized with a basic compound; the organic polymer is kneaded together with a colorant such as a self-dispersing organic pigment or a self-dispersing carbon black in an aqueous medium; and the pH of the organic polymer is neutralized or acidified using an acidic compound so that the organic polymer is deposited and fixed to the colorant. Then, a part or all of the anionic groups of the resulting hydrated cake are neutralized with a basic compound so that the colorant is microencapsulated. As a result, an aqueous dispersion liquid containing fine microencapsulated anionic pigment is produced.

As a solvent in the above described microencapsulation methods, the following substances may be used: an alkyl alcohol such as methanol, ethanol, propanol, or butanol; an aromatic hydrocarbon such as benzole, toluole, or xylole; an ester such as methyl acetate, ethyl acetate, or butyl acetate; a chlorinated hydrocarbon such as chloroform or ethylene dichloride; a ketone such as acetone or methyl isobutyl ketone; an ether such as tetrahydrofuran or dioxane; and a cellosolve such as methyl cellosolve or butyl cellosolve. Microcapsules prepared as described above are separated from the solvent by centrifugation or filtration. The separated microcapsules are stirred together with water and a solvent to form a recording liquid. The average particle diameter of a microencapsulated pigment prepared as described above is preferably between 50 and 180 nm.

Next, block patterns (basic patterns) constituting the test pattern <b>400</b> are described with reference to <figref idrefs="DRAWINGS">FIGS. 27A through 27D</figref>. Each of the block patterns is composed of line patterns and used as a minimum unit for detecting the positional deviation of liquid droplets. In a method of adjusting landing positions of liquid droplets according to an embodiment of the present invention, a reference line pattern is formed along the sub-scanning direction (paper-conveying direction) of a conveyor belt with a reference recording head (or color), and similar line patterns are formed with other recording heads (or colors) at intervals in a direction orthogonal to the sub-scanning direction. The positional deviation of liquid droplets is detected based on the distance between the reference line pattern (or the reference recording head) and each of other line patterns (or other recording heads).

<figref idrefs="DRAWINGS">FIG. 27A</figref> shows a first block pattern composed of a line pattern FK<b>1</b> formed by the recording head <b>24</b><i>k</i><b>1</b> and a line pattern FK<b>2</b> formed by the recording head <b>24</b><i>k</i><b>2</b> during the forward scan (a first scan) of the carriage <b>23</b>. With the first block pattern, positional deviation of the line pattern FK<b>2</b> is detected with reference to the line pattern FK<b>1</b>. <figref idrefs="DRAWINGS">FIG. 27B</figref> shows a second block pattern composed of a line pattern BK<b>1</b> formed by the recording head <b>24</b><i>k</i><b>1</b> and a line pattern BK<b>2</b> formed by the recording head <b>24</b><i>k</i><b>2</b> during the backward scan (a second scan). With the second block pattern, positional deviation of the line pattern BK<b>2</b> is detected with reference to the line pattern BK<b>1</b>. <figref idrefs="DRAWINGS">FIG. 27C</figref> shows a third block pattern composed of line patterns FK<b>1</b> formed by the recording head <b>24</b><i>k</i><b>1</b> and line patterns FC, FM, and FY (cyan, magenta, and yellow) formed by the corresponding recording heads <b>25</b><i>c</i>, <b>24</b><i>m</i>, and <b>24</b><i>y </i>during the forward scan (a third scan). With the third block pattern, respective positional deviation of the line patterns FC, FM, and FY is detected with reference to the corresponding line patterns FK<b>1</b>. <figref idrefs="DRAWINGS">FIG. 27D</figref> shows a fourth block pattern composed of line patterns FK<b>1</b> formed by the recording head <b>24</b><i>k</i><b>1</b> and line patterns FC, FM, and FY (cyan, magenta, and yellow) formed by the corresponding recording heads <b>25</b><i>c</i>, <b>24</b><i>m</i>, and <b>24</b><i>y </i>during the backward scan (a fourth scan). With the fourth block pattern, respective positional deviation of the line patterns FC, FM, and FY is detected with reference to the corresponding line patterns FK<b>1</b>. Various test patterns can be formed by combining the four block patterns described above.

An exemplary monochrome line misalignment test pattern and exemplary color misalignment test patterns composed of the above block patterns are described below with reference to <figref idrefs="DRAWINGS">FIGS. 28</figref>, <b>29</b>A, and <b>29</b>B.

<figref idrefs="DRAWINGS">FIG. 28</figref> shows a line misalignment test pattern <b>400</b>B including a line pattern FK<b>1</b> formed by the forward scan, a line pattern BK<b>1</b> formed by the backward scan, a line pattern FK<b>2</b> formed by the forward scan, and a line pattern BK<b>2</b> formed by the backward scan. The line patterns BK<b>1</b>, FK<b>2</b>, and BK<b>2</b> are formed at predetermined distances from the line pattern FK<b>1</b>. With the line misalignment test pattern <b>400</b>B, positional deviation of the line patterns BK<b>1</b>, FK<b>2</b>, and BK<b>2</b> can be detected with reference to the position of the line pattern FK<b>1</b>. In this example, it is assumed that the line misalignment test pattern <b>400</b>B is scanned by the image sensor <b>401</b> in one direction only.

<figref idrefs="DRAWINGS">FIGS. 29A and 29B</figref> show a color misalignment test pattern <b>400</b>C<b>1</b> and a color misalignment test pattern <b>400</b>C<b>2</b>, respectively. Each of the color misalignment test patterns <b>400</b>C<b>1</b> and <b>400</b>C<b>2</b> includes line patterns FK<b>1</b> and color line patterns FY, FM, and FC formed at predetermined distances from the corresponding line patterns FK<b>1</b>. With the color misalignment test patterns <b>400</b>C<b>1</b> and <b>400</b>C<b>2</b>, positional deviation of the line patterns FY, FM, and FC can be detected with reference to the positions of the corresponding line patterns FK<b>1</b>. In this example, it is assumed that each of the color misalignment test patterns <b>400</b>C<b>1</b> and <b>400</b>C<b>2</b> is scanned by the image sensor <b>401</b> in one direction only.

Next, an exemplary arrangement of test patterns on a conveyor belt is described with reference to <figref idrefs="DRAWINGS">FIG. 30</figref>.

Here, a direction of movement of the carriage <b>23</b> from the back side toward the front side of the image forming apparatus <b>200</b> shown in <figref idrefs="DRAWINGS">FIG. 2</figref> is called a forward direction and a direction from the front side toward the back side is called a backward direction. Also, it is assumed that the recording heads <b>24</b><i>c</i>, <b>24</b><i>k</i><b>1</b>, <b>24</b><i>k</i><b>2</b>, <b>24</b><i>m</i>, and <b>24</b><i>y </i>are arranged in the forward direction in the order mentioned.

In <figref idrefs="DRAWINGS">FIG. 30</figref>, line misalignment test patterns <b>400</b>B<b>1</b> and <b>400</b>B<b>2</b> are formed near the corresponding sides of the conveyor belt <b>31</b>, and color misalignment test patterns <b>400</b>C<b>1</b> and <b>400</b>C<b>2</b> are formed approximately in the middle of the conveyor belt <b>31</b>. In other words, in this example, test patterns are formed within a printing area of the conveyor belt <b>31</b> and arranged in a direction orthogonal to the sub-scanning direction. Also, the test patterns are formed in areas on the conveyor belt <b>31</b> other than those where the belt surface is rough (e.g., areas where the separating claws <b>39</b>, which separate a recording medium from the conveyor belt <b>31</b>, are in contact with the belt surface).

Each of the test patterns <b>400</b>B<b>1</b>, <b>400</b>B<b>2</b>, <b>400</b>C<b>1</b>, and <b>400</b>C<b>2</b> is scanned multiple times by the image sensor <b>401</b> just after it is formed. The image sensor <b>401</b> scans each test pattern multiple times either in one direction or in both directions.

An exemplary process of adjusting landing positions of liquid droplets (landing position adjusting process) performed by the main control unit <b>310</b> according to an embodiment of the present invention is described below with reference to <figref idrefs="DRAWINGS">FIG. 31</figref>.

A landing position adjusting process is performed, for example, when cleaning K<b>1</b> or K<b>2</b> of the recording head <b>24</b><i>k</i><b>1</b> or <b>24</b><i>k</i><b>2</b>, which uses black ink, is performed, when cleaning (after-unused-period cleaning) of the recording heads <b>24</b> is performed after the image forming apparatus <b>200</b> is unused for a long time, and when the variation of the environmental temperature exceeds a predetermined value.

First, as shown in <figref idrefs="DRAWINGS">FIG. 31</figref>, cleaning of the conveyor belt <b>31</b> is performed as first preprocessing. Next, calibration of the image sensor <b>401</b> (light-emitting element <b>402</b> and light-receiving element <b>403</b>) is performed as second preprocessing so that a constant sensor output voltage is obtained from the light-emitting element <b>402</b> throughout the surface of the conveyor belt <b>31</b>.

Then, first line patterns are formed by moving the carriage <b>23</b> in the forward direction (first scan), and second line patterns are formed by moving the carriage <b>23</b> in the backward direction (second scan). The first line patterns indicate line patterns formed by the forward scan (e.g., line patterns in <figref idrefs="DRAWINGS">FIG. 30</figref> with F in their reference numbers), and the second line patterns indicate line patterns formed by the backward scan (e.g., line patterns in <figref idrefs="DRAWINGS">FIG. 30</figref> with B in their reference numbers). The first line patterns and the second line patterns constitute the test pattern <b>400</b>.

The test pattern <b>400</b> is scanned by moving the carriage <b>23</b> in the forward direction (third scan) while emitting light from the light-emitting element <b>402</b> of the image sensor <b>401</b>. The sensor output voltage from the light-receiving element <b>403</b> of the image sensor <b>401</b> is converted from analog to digital, and stored in a memory.

Then, the processing algorithm <b>526</b> is executed by the CPU <b>301</b> to calculate the amount of positional deviation of liquid droplets. For example, a difference in landing positions of liquid droplets formed in the forward scan and the backward scan, and positional deviation of color liquid droplets (or color line patters) are calculated.

More specifically, reference line patterns are formed by the forward and backward scans using a reference recording head (or color) along the sub-scanning direction of the conveyor belt <b>31</b>, and similar line patterns are formed at intervals using other recording heads. The line patterns (or the test pattern <b>400</b>) are scanned to obtain a sensor output voltage. Based on the sensor output voltage, the processing algorithm <b>526</b> calculates center points (or center lines) of the line patterns, obtains distances between the line patterns, compares the obtained distances with optimal distances between the line patterns, and thereby obtains the amounts of positional deviation of liquid droplets (or line patterns). In this embodiment, as described above, a linear encoder is used to detect the position of the carriage <b>23</b>. This makes it possible to obtain an accurate distance between line patterns by using positions of the carriage <b>23</b> at the time when liquid droplets are detected as coordinates of the liquid droplets.

Referring back to <figref idrefs="DRAWINGS">FIG. 31</figref>, after the execution of the processing algorithm <b>526</b>, the main control unit <b>310</b> determines whether the scanning result from the image sensor <b>401</b> is normal. If the scanning result is normal, the main control unit <b>310</b> determines whether scanning the test pattern <b>400</b> (pattern scanning operation) has been performed N times. If No, the main control unit <b>310</b> returns to the step of scanning the test pattern <b>400</b> (the third scan). Thus, in this example, the pattern scanning operation is performed in the forward direction N times. After the pattern scanning operation is performed N times, an adjustment value for adjusting the liquid-jet timing is calculated based on the amount of positional deviation obtained by adjusting a forward-backward difference, which is a difference in landing positions of liquid droplets formed in the forward scan and the backward scan of the carriage <b>23</b>, by the thickness of paper (or a recording medium). Then, the liquid-jet timing is adjusted based on the adjustment value. After adjusting the liquid-jet timing, cleaning of the surface of the conveyor belt <b>31</b> is performed as postprocessing.

If the scanning result from the image sensor <b>401</b> is abnormal, the main control unit <b>310</b> determines whether the retry is the first time. If the retry is the first time, the process returns to the step of scanning the test pattern <b>400</b>. If the retry is not the first time, the main control unit <b>310</b> determines whether the number of retries is smaller than a predetermined number “n”. If the number of retries is smaller than “n”, the process returns to the first preprocessing. If the number of retries is equal to or larger than “n”, the main control unit <b>310</b> performs cleaning of the conveyor belt <b>31</b> as postprocessing, and then performs error processing.

As described above, an embodiment of the present invention provides a liquid-jet device that forms a test pattern composed of separate liquid droplets on a water-repellent part, illuminates the test pattern, detects (or scans) the test pattern based on specularly reflected light from the test pattern, and adjusts landing positions of liquid droplets based on the detection result (scanning result). This configuration makes it possible to accurately detect landing positions of liquid droplets with a simple mechanism and thereby to accurately adjust landing positions of the liquid droplets.

Another embodiment of the present invention provides an image forming apparatus that includes a liquid-jet device configured as described above and that can form a high-quality image by accurately jetting liquid droplets.

A second embodiment of the present invention is described below with reference to <figref idrefs="DRAWINGS">FIG. 32</figref>.

According to the second embodiment, the image forming unit <b>2</b> of the image forming apparatus <b>200</b> includes two image sensors <b>401</b> attached to a sensor support <b>800</b> disposed between the front board <b>101</b>F and the rear board <b>101</b>R. This configuration makes it possible to scan the test pattern <b>400</b> without being affected by the vibration of the carriage <b>23</b>. This configuration can also be applied to a line-type image forming apparatus including a line-type recording head.

A third embodiment of the present invention is described below with reference to <figref idrefs="DRAWINGS">FIG. 33</figref>.

An image forming apparatus of the third embodiment includes, instead of a conveyor belt, a conveyor roller <b>801</b> that conveys a recording medium (or a paper sheet) placed on or wound around it. In the image forming apparatus of the third embodiment, liquid droplets <b>500</b> are jetted onto the upper edge of the conveyor roller <b>801</b> such that the liquid droplets <b>500</b> are positioned at equal distances from the recording heads <b>24</b> (to be precise, from the image sensor <b>401</b>). With this configuration, the proportion of specularly reflected light from areas where the liquid droplets <b>500</b> are not present is large, and the proportion of specularly reflected light from areas where the liquid droplets are present is small. Therefore, this configuration also makes it possible to accurately detect landing positions of liquid droplets.

A fourth embodiment of the present invention is described below with reference to <figref idrefs="DRAWINGS">FIGS. 34 through 37</figref>. <figref idrefs="DRAWINGS">FIG. 34</figref> is a drawing illustrating the imaging engine unit <b>100</b> of the fourth embodiment. <figref idrefs="DRAWINGS">FIG. 35</figref> is another drawing illustrating the imaging engine unit <b>100</b> of the fourth embodiment. <figref idrefs="DRAWINGS">FIGS. 36A and 36B</figref> are drawings illustrating a retracting mechanism according to the fourth embodiment. <figref idrefs="DRAWINGS">FIG. 37</figref> is a flowchart showing an exemplary process according to the fourth embodiment.

The imaging engine unit <b>100</b> of the fourth embodiment includes a cleaning part (cleaning unit) <b>901</b> for removing test patterns from the surface of the conveyor belt <b>31</b>. The cleaning part <b>901</b> is brought into contact with and retracted from the surface of the conveyor belt <b>31</b> by a retracting mechanism <b>902</b>.

As the material of the cleaning part <b>901</b>, a porous material, such as polyvinyl alcohol (PVA) sponge, that can absorb liquids such as ink is preferably used. As shown in <figref idrefs="DRAWINGS">FIGS. 36A and 36B</figref>, the retracting mechanism <b>902</b> includes a solenoid <b>903</b>, an arm <b>905</b> swingably supported in the middle by a spindle <b>904</b>, and a tension spring <b>906</b>. One end of the arm <b>905</b> is connected to a plunger <b>903</b><i>a </i>of the solenoid <b>903</b>, and the cleaning part <b>901</b> is attached to the other end of the arm <b>905</b>. The tension spring <b>906</b> is interposed between a locking part <b>907</b> of the arm <b>905</b> and an anchor <b>908</b>. A combination of a motor and a cam may be used instead of the solenoid <b>903</b>.

When the solenoid <b>903</b> is not energized, the plunger <b>903</b><i>a </i>protrudes as shown in <figref idrefs="DRAWINGS">FIG. 36A</figref> and causes the cleaning part <b>901</b> to be retracted from the surface of the conveyor belt <b>31</b> as shown in <figref idrefs="DRAWINGS">FIG. 34</figref>. When the solenoid <b>903</b> is energized, the plunger <b>903</b><i>a </i>retreats, causes the arm <b>905</b> to swing as shown in <figref idrefs="DRAWINGS">FIG. 36B</figref>, and thereby causes the cleaning part <b>901</b> to be pressed against the surface of the conveyor belt <b>31</b> as shown in <figref idrefs="DRAWINGS">FIG. 35</figref>.

As shown in <figref idrefs="DRAWINGS">FIG. 37</figref>, after the test pattern <b>400</b> is formed on the conveyor belt <b>31</b>, the conveyor belt <b>31</b> is turned until the test pattern <b>400</b> reaches the scanning position of the image sensor <b>401</b>. Next, a driving unit (the sub-scanning motor <b>131</b>) of the conveyor belt <b>31</b> is stopped and the cleaning part <b>901</b> is pressed against the conveyor belt <b>31</b> by driving the retracting mechanism <b>902</b>. Then, with the cleaning part <b>901</b> being pressed against the conveyor belt <b>31</b>, the test pattern <b>400</b> is scanned with the image sensor <b>401</b>.

After scanning the test pattern <b>400</b>, the conveyor belt <b>31</b> is turned to remove the test pattern <b>400</b> with the cleaning part <b>901</b>. Then, the driving unit of the conveyor belt <b>31</b> is stopped, and the cleaning part <b>901</b> is retracted from the conveyor belt <b>31</b> by driving the retracting mechanism <b>902</b>.

Thus, the cleaning part <b>901</b> cleans the conveyor belt <b>31</b> and also holds the conveyor belt <b>31</b> when the test pattern <b>400</b> is scanned. This configuration prevents a recording medium from being smeared by the test pattern <b>400</b> or ink adhering to the conveyor belt <b>31</b>. Also, this configuration prevents the conveyor belt <b>31</b>, where the test pattern <b>400</b> is formed, from being stained and thereby improves the accuracy of detecting the test pattern <b>400</b>. Further, this configuration prevents vibration of the conveyor belt <b>31</b> when the test pattern <b>400</b> is scanned and thereby improves the accuracy of adjusting landing positions of liquid droplets.

A fifth embodiment of the present invention is described below with reference to <figref idrefs="DRAWINGS">FIG. 38</figref>. <figref idrefs="DRAWINGS">FIG. 38</figref> is a drawing illustrating the fifth embodiment of the present invention.

In the fifth embodiment, the cleaning part <b>901</b> is placed in a position facing the driven roller <b>33</b>. With this configuration, the conveyor belt <b>31</b> is sandwiched between the cleaning part <b>901</b> and the driven roller <b>33</b>, and cannot escape when pressed by the cleaning part <b>901</b>. Therefore, this configuration makes it possible to firmly press the cleaning part <b>901</b> against the conveyor belt <b>31</b>. As an alternative configuration, the cleaning part <b>901</b> may be placed in a position facing the conveying roller (drive roller) <b>32</b>.

A sixth embodiment of the present invention is described below with reference to <figref idrefs="DRAWINGS">FIG. 39</figref>. <figref idrefs="DRAWINGS">FIG. 39</figref> is a drawing illustrating the sixth embodiment of the present invention.

In the sixth embodiment, the cleaning part <b>901</b> is placed in a position facing the platen guide <b>35</b>. This configuration provides advantageous effects similar to those of the fifth embodiment.

A seventh embodiment of the present invention is described below with reference to <figref idrefs="DRAWINGS">FIG. 40</figref>. <figref idrefs="DRAWINGS">FIG. 40</figref> is a drawing illustrating the seventh embodiment of the present invention.

In the seventh embodiment, the cleaning part <b>901</b> is placed upstream of the paper dust removing part <b>191</b>. If the paper dust removing part <b>191</b> is smeared by the ink of the test pattern <b>400</b>, its cleaning performance is reduced, and also the smeared paper dust removing part <b>191</b>, in turn, smears the conveyor belt <b>31</b>. Placing the cleaning part <b>901</b> upstream of the paper dust removing part <b>191</b> makes it possible to prevent this problem.

As the conveyor belt <b>31</b> in the above embodiments, any conveyor belt that holds paper or a recording medium by stiction, air suction, electrostatic attraction, or their combination may be used.

An eighth embodiment of the present invention is described below with reference to <figref idrefs="DRAWINGS">FIG. 41</figref>. <figref idrefs="DRAWINGS">FIG. 41</figref> is a drawing illustrating the eighth embodiment of the present invention.

In the eighth embodiment, the cleaning part <b>901</b> is placed upstream of the charging roller <b>34</b> for charging the conveyor belt <b>31</b>. If the charging roller <b>34</b> is smeared by the ink of the test pattern <b>400</b>, its charging performance is reduced, and also the smeared charging roller <b>34</b>, in turn, smears the conveyor belt <b>31</b>. Placing the cleaning part <b>901</b> upstream of the charging roller <b>34</b> makes it possible to prevent this problem.

A ninth embodiment of the present invention is described below with reference to <figref idrefs="DRAWINGS">FIG. 42</figref>. <figref idrefs="DRAWINGS">FIG. 42</figref> is a flowchart showing an exemplary process according to the ninth embodiment.

In the ninth embodiment, as shown in <figref idrefs="DRAWINGS">FIG. 42</figref>, after the test pattern <b>400</b> is formed on the conveyor belt <b>31</b>, the conveyor belt <b>31</b> is turned until the test pattern <b>400</b> reaches the scanning position of the image sensor <b>401</b>. Next, the driving unit of the conveyor belt <b>31</b> is stopped and the cleaning part <b>901</b> is pressed against the conveyor belt <b>31</b> with a pressing force A by driving the retracting mechanism <b>902</b>. Then, with the cleaning part <b>901</b> being pressed against the conveyor belt <b>31</b>, the test pattern <b>400</b> is scanned with the image sensor <b>401</b>.

After scanning the test pattern <b>400</b>, the pressing force A being applied to the cleaning part <b>901</b> is changed to a pressing force B (B&lt;A), and the conveyor belt <b>31</b> is turned to remove the test pattern <b>400</b> with the cleaning part <b>901</b>. Then, the driving unit of the conveyor belt <b>31</b> is stopped, and the cleaning part <b>901</b> is retracted from the conveyor belt <b>31</b> by driving the retracting mechanism <b>902</b>.

When the cleaning part <b>901</b> is pressed against the conveyor belt <b>31</b> being turned to remove the test pattern <b>400</b>, the cleaning part <b>901</b> may damage the conveyor belt <b>31</b>. In this embodiment, to reduce the damage, the pressing force A is used to press the cleaning part <b>901</b> against the conveyor belt <b>31</b> to hold and stabilize the conveyor belt <b>31</b> while the test pattern <b>400</b> is being scanned, and the pressing force B, which is weaker than the pressing force A and is still sufficient to remove the test pattern <b>400</b>, is used during cleaning.

The pressing force applied to the cleaning part <b>901</b> can be easily changed by varying the electric current supplied to the solenoid <b>903</b> of the retracting mechanism <b>902</b>.

A tenth embodiment of the present invention is described below with reference to <figref idrefs="DRAWINGS">FIG. 43</figref>. <figref idrefs="DRAWINGS">FIG. 43</figref> is a drawing illustrating the tenth embodiment of the present invention.

The imaging engine unit <b>100</b> of the tenth embodiment includes a cleaning roller (cleaning unit) <b>911</b> instead of the cleaning part <b>901</b> and a motor <b>912</b> used as a driving unit for rotating the cleaning roller <b>911</b> via a belt <b>913</b>. The cleaning roller <b>911</b> is moved along with the motor <b>912</b> by the retracting mechanism <b>902</b>. As the material of the cleaning roller <b>911</b>, a porous material, such as PVA sponge, that can absorb liquids such as ink is preferably used.

As shown in <figref idrefs="DRAWINGS">FIG. 44</figref>, after the test pattern <b>400</b> is formed on the conveyor belt <b>31</b>, the conveyor belt <b>31</b> is turned until the test pattern <b>400</b> reaches the scanning position of the image sensor <b>401</b>. Next, the driving unit of the conveyor belt <b>31</b> is stopped and the cleaning roller <b>911</b> is pressed against the conveyor belt <b>31</b> by driving the retracting mechanism <b>902</b>. Then, with the cleaning roller <b>911</b> being pressed against the conveyor belt <b>31</b>, the test pattern <b>400</b> is scanned with the image sensor <b>401</b>.

After scanning the test pattern <b>400</b>, the cleaning roller <b>911</b> is rotated (preferably in a direction opposite to the moving direction of the conveyor belt <b>31</b>) and the conveyor belt <b>31</b> is turned to remove the test pattern <b>400</b> with the cleaning roller <b>911</b>. Then, after stopping the driving unit of the conveyor belt <b>31</b> and the rotation of the cleaning roller <b>911</b>, the cleaning roller <b>911</b> is retracted from the conveyor belt <b>31</b> by driving the retracting mechanism <b>902</b>.

Using the cleaning roller <b>911</b>, which is rotated in a direction opposite to the moving direction of the conveyor belt <b>31</b>, instead of the cleaning part <b>901</b> makes it possible to more efficiently remove the test pattern <b>400</b> from the conveyor belt <b>31</b>. Meanwhile, the cleaning roller <b>911</b> is preferably not rotated when it is pressed against the conveyor belt <b>31</b> to hold the belt during the step of scanning the test pattern <b>400</b>. This makes it possible to prevent the cleaning roller <b>911</b> from vibrating the conveyor belt <b>31</b>.

As a driving unit for the cleaning roller <b>911</b>, a stepping motor is preferably used. When pressing the cleaning roller <b>911</b> against the conveyor belt <b>31</b> to hold the belt during the step of scanning the test pattern <b>400</b>, it is preferable to completely stop the rotation of the cleaning roller <b>911</b> by exciting the stepping motor.

The cleaning part <b>901</b> and the cleaning roller <b>911</b> are just examples of a cleaning unit for removing a test pattern from a conveyor belt, and other types of cleaning units may also be used.

An eleventh embodiment of the present invention is described below with reference to <figref idrefs="DRAWINGS">FIG. 45</figref>. <figref idrefs="DRAWINGS">FIG. 45</figref> is a perspective view of a cleaning roller according to the eleventh embodiment of the present invention.

According to the eleventh embodiment, the cleaning roller <b>911</b> described in the tenth embodiment comprises a first roller part <b>911</b><i>a </i>disposed in the middle of the cleaning roller <b>911</b> and made of an ink-absorbent material for removing the test pattern <b>400</b>, and second roller parts <b>911</b><i>b </i>disposed at the ends of the cleaning roller <b>911</b> and made of a material with a high friction coefficient suitable for holding the conveyor belt <b>31</b>.

This configuration is suitable for a case where the test pattern <b>400</b> is formed substantially in the middle of (not near the edges of) the conveyor belt <b>31</b> with respect to the main-scanning direction. In this case, the first roller part <b>911</b><i>a </i>can efficiently remove the test pattern <b>400</b> and the second roller parts <b>911</b><i>b </i>can effectively hold the conveyor belt <b>31</b>.

A twelfth embodiment of the present invention is described below with reference to <figref idrefs="DRAWINGS">FIG. 46</figref>. <figref idrefs="DRAWINGS">FIG. 46</figref> is a flowchart showing an exemplary process according to the twelfth embodiment.

In this exemplary process, the conveyor belt <b>31</b> is stopped first, and the cleaning part <b>901</b> (or the cleaning roller <b>911</b>) is pressed against the conveyor belt <b>31</b> by driving the retracting mechanism <b>902</b>. Next, the test pattern <b>400</b> is formed on the conveyor belt <b>31</b>, the cleaning part <b>901</b> is retracted from the conveyor belt <b>31</b> by driving the retracting mechanism <b>902</b>, the conveyor belt <b>31</b> is turned until the test pattern <b>400</b> reaches the scanning position of the image sensor <b>401</b>, the driving unit of the conveyor belt <b>31</b> is stopped, and the test pattern <b>400</b> is scanned by the image sensor <b>401</b>.

After scanning the test pattern <b>400</b>, the cleaning part <b>901</b> (or the cleaning roller <b>911</b>) is pressed against the conveyor belt <b>31</b> again by driving the retracting mechanism <b>902</b>, the conveyor belt <b>31</b> is turned to remove the test pattern <b>400</b> with the cleaning part <b>901</b>. Then, the driving unit of the conveyor belt <b>31</b> is stopped, and the cleaning part <b>901</b> is retracted from the conveyor belt <b>31</b> by driving the retracting mechanism <b>902</b>.

Thus, in this embodiment, the conveyor belt <b>31</b> is held by the cleaning part <b>901</b> (or the cleaning roller <b>911</b>) while the test pattern <b>400</b> is formed. This configuration reduces vibration of the conveyor belt <b>31</b> and thereby makes it possible to accurately form the test pattern <b>400</b>.

A thirteenth embodiment of the present invention is described below.

If the image scanning unit <b>11</b> of the image forming apparatus <b>200</b> is operated while a landing position adjusting process is being performed, the vibration of the image scanning unit <b>11</b> may cause the conveyor belt <b>31</b> to vibrate and thereby influence the result of the landing position adjusting process. In the thirteenth embodiment, to obviate or reduce this problem, the conveyor belt <b>31</b> is held and stabilized by the cleaning part <b>901</b> (or the cleaning roller <b>911</b>) if the test pattern <b>400</b> is scanned during a scanning process of the image scanning unit <b>11</b>.

A fourteenth embodiment of the present invention is described below.

In the fourteenth embodiment, the image forming apparatus <b>200</b> includes a vibration detecting unit (not shown) for detecting vibration and is configured to hold and stabilize the conveyor belt <b>31</b> with the cleaning part <b>901</b> (or the cleaning roller <b>911</b>) when the vibration detected by the vibration detecting unit exceeds a predetermined value. This configuration makes it possible to accurately perform a landing position adjusting process.

A fifteenth embodiment of the present invention is described below with reference to <figref idrefs="DRAWINGS">FIG. 47</figref>. <figref idrefs="DRAWINGS">FIG. 47</figref> is a flowchart showing an exemplary process according to the fifteenth embodiment.

In the fifteenth embodiment, the user turns the conveyor belt <b>31</b> manually to move a smeared portion of the conveyor belt <b>31</b> to a position suitable for cleaning, and operates an operations unit (operations panel) to press the cleaning part <b>901</b> (or the cleaning roller <b>911</b>) against the conveyor belt <b>31</b> and thereby to hold the conveyor belt <b>31</b>. Then, the user cleans the smeared portion of the conveyor belt <b>31</b> manually. Holding the conveyor belt <b>31</b> by the cleaning part <b>901</b> (or the cleaning roller <b>911</b>) makes it easier for the user to clean the conveyor belt <b>31</b>. After cleaning the conveyor belt <b>31</b>, the user operates the operations unit again to retract the cleaning part <b>901</b> (or the cleaning roller <b>911</b>) from the conveyor belt <b>31</b>.

A sixteenth embodiment of the present invention is described below with reference to <figref idrefs="DRAWINGS">FIGS. 48 and 49</figref>. <figref idrefs="DRAWINGS">FIG. 48</figref> is a drawing illustrating the sixteenth embodiment of the present invention. <figref idrefs="DRAWINGS">FIG. 49</figref> is a flowchart showing an exemplary process according to the sixteenth embodiment.

In the sixteenth embodiment, the imaging engine unit <b>100</b> includes a smear detection sensor <b>915</b> for detecting a smear on the conveyor belt <b>31</b>. For example, a laser micrometer may be used as the smear detection sensor <b>915</b>. The smear detection sensor <b>915</b> continuously monitors the thickness of the conveyor belt <b>31</b>, detects the amount of adhering ink (or liquid) on the conveyor belt <b>31</b> based on the increase in thickness of the conveyor belt <b>31</b> from the initial value, and outputs a signal if the detected amount of adhering ink exceeds a predetermined value or a level that is difficult to remove with the cleaning part <b>901</b> (or the cleaning roller <b>911</b>).

As shown in <figref idrefs="DRAWINGS">FIG. 49</figref>, if the detected amount of adhering ink exceeds the predetermined value, the main control unit <b>310</b> of the image forming apparatus <b>200</b> stops image forming operations and turns the conveyor belt <b>31</b> to move its smeared portion to a position convenient for the user or a serviceperson to perform cleaning (e.g., to a position where the smeared portion is exposed when a cover is opened). Then, the main control unit <b>310</b> presses the cleaning part <b>901</b> (or the cleaning roller <b>911</b>) against the conveyor belt <b>31</b>, and the user or the serviceperson cleans the conveyor belt <b>31</b> manually. Holding the conveyor belt <b>31</b> by the cleaning part <b>901</b> (or the cleaning roller <b>911</b>) makes it easier for the user or the serviceperson to clean the conveyor belt <b>31</b>. After cleaning the conveyor belt <b>31</b>, the user operates an operations unit (not shown) to retract the cleaning part <b>901</b> (or the cleaning roller <b>911</b>) from the conveyor belt <b>31</b>.

A seventeenth embodiment of the present invention is described below with reference to <figref idrefs="DRAWINGS">FIG. 50</figref>. <figref idrefs="DRAWINGS">FIG. 50</figref> is a flowchart showing an exemplary process according to the seventeenth embodiment.

In this exemplary process, the conveyor belt <b>31</b> is stopped first, and the cleaning part <b>901</b> (or the cleaning roller <b>911</b>) is pressed against the conveyor belt <b>31</b> by driving the retracting mechanism <b>902</b>. Next, the test pattern <b>400</b> is formed on the conveyor belt <b>31</b>, the cleaning part <b>901</b> (or the cleaning roller <b>911</b>) is retracted from the conveyor belt <b>31</b> by driving the retracting mechanism <b>902</b>, the conveyor belt <b>31</b> is turned until the test pattern <b>400</b> reaches the scanning position of the image sensor <b>31</b>, the driving unit of the conveyor belt <b>31</b> is stopped, and the cleaning part <b>901</b> (or the cleaning roller <b>911</b>) is pressed against the conveyor belt <b>31</b> again by driving the retracting mechanism <b>902</b>. Then, the test pattern <b>400</b> is scanned with the image sensor <b>401</b>.

After scanning the test pattern <b>400</b>, the conveyor belt <b>31</b> is turned to remove the test pattern <b>400</b> with the cleaning part <b>901</b> (or the cleaning roller <b>911</b>). Then, the driving unit of the conveyor belt <b>31</b> is stopped, and the cleaning part <b>901</b> is retracted from the conveyor belt <b>31</b> by driving the retracting mechanism <b>902</b>.

Thus, in this embodiment, the conveyor belt <b>31</b> is held by the cleaning part <b>901</b> (or the cleaning roller <b>911</b>) while the test pattern <b>400</b> is formed and scanned. This configuration reduces vibration of the conveyor belt <b>31</b> and thereby makes it possible to accurately form and scan the test pattern <b>400</b>.

Thus, embodiments of the present invention make it possible to accurately detect a test pattern composed of liquid droplets and thereby to accurately adjust landing positions of liquid droplets.

Embodiments of the present invention provide a liquid-jet device, an image forming apparatus, and a method for adjusting landing positions of liquid droplets where a test pattern composed of separate liquid droplets are formed on a water-repellent part, the test pattern is detected by illuminating the test pattern and receiving specularly reflected light from the test pattern, and landing positions of liquid droplets are adjusted based on the detection result (scanning result). This configuration makes it possible to accurately detect landing positions of liquid droplets with a simple mechanism and thereby to accurately adjust the landing positions of the liquid droplets.

The present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention.

The present application is based on Japanese Priority Application No. 2007-069688, filed on Mar. 17, 2007, the entire contents of which are hereby incorporated herein by reference.